Surprise, I'm voting ban.
I don't actually think Volcarona is the most broken thing in the tier right now, I actually think just behind Waterpon and Kyurem. However, it's still broken.
Volc has always been questionable and has always been seen as the Match-Up Moth, but I think its base kit essentially gives it the tools to use tera as effectively as it does to amplify what it's always been able to do. Before tera, volc has a few things that mean it's able to snowball; It has quiver dance, it has flame body which pairs very well with its typing, it has great stats, boots, fiery dance and reliable recovery. These are all an important part of the picture of what makes Volc broken. Quiver Dance is a stupid move that boosts its offenses, defenses, and speed all in one. It being special instead of physical is important because you can't use rocky helmet chip or intimidate to slow it down like you can with DD users like RM and Dnite. It's an insane snowball tool because it improves your mu into offensive teams and defensive teams all in one; the only other things with QD off the top of my head is Lilligant (bad stats, no coverage) and Frosmoth (ice/bug lmao), neither of which have the tools to make use of QD like volc can. The SpD boost volc gets make it harder to revenge kill on the special side with its already solid special bulk, or even in general with its alright speed. QD makes both of these harder, and so killing it on the physical side would be fine, except for the second problem; it gets flame body. Flame body is dumb as hell on volc because of certain lines in play; it both gives volc a much easier time setting up and makes it harder to revenge. Volc's physical defense is usually worse then it's special defense, and so it often becomes fairly easy to throw it out into physical attackers in the hopes that it lands a burn. If it does, not only is your physical attacker crippled for the rest of the game, but it now probably has to switch out because it likely can't outdamage any healing volc decides to do or can't revenge it before volc hits it with fiery dance or sets up on it. This means that a burn proc gives Volc a much easier time setting up because it negates its less bulky physical side, which it can't boost with QD. Similarly, Volc now becomes harder to revenge kill because of the chance of a burn proc on physical attacks means your hesistant to throw out a physical move onto its less bulky side because even if it hits, if you burn your revenge killing attempt not only might be ruined and so volc can keep setting up, but you also cripple your mon for the rest of the game. Flame body is RNG reliant, but it pairs extemely well with QD because it can functionally give volc way more physical bulk. It's typing would be dogshit because rocks is everywhere, but Volc can just throw on boots, and unlike something like Moltres or Talonflame, even IF its knocked off, it can still just go for a sweep with flame body and fiery dance cheese anyways. Sure, on balance it wants its defensive presence, but on somehting more offensive there's no downside to it trying to blow a hole in the enemy team with a sweep the second it gets knocked.
Finally, it has fiery dance, which is also stupid because it has a 50% chance to boost your SpA on a mon who is already known for snowballing. If you switch in a defensive answer, Volc proccing the fiery dance boost means that your answer is now way less reliable, and thats also forgetting that this can happen MULTIPLE TIMES IN A ROW; Volc has a 25% if it clicks Fiery Dance twice to hit +3 spA, +1 SpD, +1 Speed after a QD, which is insane and makes defensive counterplay (because you're not outspeeding it at +1 unless your a scarfer/booster speed/grasspon tera) much harder. As an example, if Pult switches in on Offensive Volc when it QDs, Volc actually has a 50% chance of beating it even without tera-ing because it can coin flip for the fiery dance boost, and if that hits, can guaranteed finish it off with bug buzz. That's also forgetting burn procs, which depending on how often you throw it out on uturns, can also net multiple burns. Now, yes, this relies on RNG, and the chance of hitting +3 SpA while also burning multiple enemy pokemon is low; however, the point is that Volc has the tools to cheese its counterplay and make snowballing much easier ALREADY, before discussing tera.
When adding in Tera, Volc gets to pick and choose its counterplay entirely. Volc's big issues are that it's typing is incredible rocks-weak and it doesn't have fantastic coverage; boots really helps with the rocks-weakness (functionally entirely on offense) and tera not only means it can avoid rocks but similarly gives it another turn to setup/attempt a sweep AND fixes its coverage issue. Things like Heatran or Dirge which would blank it before now get smoked by tera-ground. Mons like Dnite/Pult/GF lose to Tera-Dragon, which also solves its water weakness. Stuff like Prim and Ting Lu gets smoked by tera grass giga drain. Something like Garg can't threaten it nearly as well once it tera's because salt cure now does minimal damage and it might either have giga drain or morning sun. This is important because this defensive counterplay getting smoked means that not only is your team now much more vulnerable to other threats on the offenses volc finds itself on, but because of how hard it is to offensively revenge kill it thanks to QD/Flame body. There is a reason this is one of the only mons that consistently runs tera-blast, an otherwise dead moveslot; because you get so much value from tera-ing that it's worth it. It's also worth noting that prior to this gen, Fiery Dance was not in the recommended movesets for Volcarona at all, and now it is; because tera means it can leverage its snowball potential to win games on the spot. Now, Volc cannot beat all its checks with one set, but the point is that not only can it cheese its way to a sweep even without tera (thereby potentially not needing it) but that you don't know which set its running until it might've procced some burns, gotten a QD boost and a fiery dance boost on top of that. If I switch toxic glowking into it after a QD boost and it clicks sub, my defensive counterplay gets a lot harder, especially because it can continue to snowball to be harder to revenge kill and it can tera and go for the sweep. Yes, just because one mon's set has a good matchup into a team doesn't make it broken, but Volc's capacity to snowball and the lack of knowledge surrounding tera and sets means a single mistake can allow the volc to get lucky and pull off a sweep or just demolish your team. This is unique to Volc; no other mon either uses tera or tera blast this way or can hax like it because of the sheer amount of snowballing tools it has pre-tera.
Volc does have counterplay; CM Blissey and Clod are the only counters but these only fit on hard stall, so they're not splashable at all. Other then that, it has checks, and that's it. Skeledirge and Heatran exist but both get smoked by ground and neither are all that splashable, and heatran kinda sucks if I'm being honest (your counterplay as heatran is pray magma storm hits). Dirge is better but hard to fit when you're already struggling to cover as many threats on balance as you are. Dragons like Dnite, Pult, Bolt and Gouging Fire exist but the former two lose to wisp sets even without tera, Pult is frail enough where it gets cheesed by offensive volc anyways, and they all lose to tera dragon and only Dnite doesn't lose to tera ground. Waters like Prim exist but it needs to be AV to actually stand a chance (dogshit) and also can just lose to giga drain anyways. Garg can check it but counterplay becomes a lot harder if its giga drain volc, or if it teras at all to be honest because then Garg can't put as much pressure on it with Salt Cure and it can boost/heal up in its face. Ting Lu can phaze it but again, gets cooked by Giga Drain OR Bug Buzz and phazing it can help but is a temporary stopgap more then anything; it just means you have to face the volc sweep later in the game. GKing with Psyshock and Toxic is also great counterplay, but it NEEDS those two moves to actually reliably check it; toxic gets cooked by any substitute set, ausma has mentioned how twave just means it keeps boosting in its face. Importantly though, GKing is already one of the only things stapling balance together right now and outside of the mandatory CR it has three moveslots that are heavily being competed for. It needs psyshock and toxic to reliably check Volc, but it ALSO wants twave for faster threats, future sight for breaking, sludge bomb for fairies/poison fishes, ice beam for grounds/gliscor/dragons, flamethrower/focus blast for steels, slack off for recovery and more. It's already forced to do a TON on teams anyways, and the only way it reliably beats Volc is by devoting 2/3 of its available moveslots to it. That's not healthy, and to be honest, I would argue that devoting those moveslots for Volcarona is close to mandatory right now with how prevalent it is; wihtout them, it can get cheesed and outright lose. It has already been commented on that we lack great fire resists right now for special mons, and Volc exacerbates this by either being able to cheese all the available ones so it can spam fiery dance and snowball even more.
Now for some counterarguments:
"Volc is stopping Kyurem, Valiant and Enamorus from running wild and provides valuable defensive utilty to the tier": I don't care. This is not only barely true but frankly isn't an argument about Volc's brokenness at all. Volc loses to specs Kyu draco outright, it loses to the best val set, being SD, so it actually DOESN'T keep it in check at all outside of mixed and CM sets, and Enamorus is whatever. If you need something for Kyurem or Fairies, glowking is right there.
Importantly though, tiering isn't a pros and cons list, to reference Finch, its about banning something if it's broken. These arguments are therefore non-starters; they do not touch on things that matter for tiering at all. If Valiant or Enamorus are broken (or problematic) after the Volc ban, they will be looked at. It quite literally has no bearing in this debate. It's really just fearmongering, which has no place in this thread.
This is not mentioning Rillaboom, which.... ???? Rillaboom isn't broken at all, GTerrain was only a problem with Sneasler, there are a plethora of grass resists in OU. Rillaboom is great but it's not the threat certain users think it is.
TLDR it was always stupid and tera lets it leverage that. Ban.