Diglett and Trapinch are individually superbly strong, potentially suspect-worthy threats, but suspecting Arena Trap because they're too much when you put them together doesn't make any sense. These are not Pokemon that you would ever want to run together on a serious team - you'll either run Diglett, or you'll run Trapinch. It doesn't matter that they have different checks, because they will not be used together; they are separate (but similar) threats to prepare for in the teambuilding stage. Looking at Diglett and Trapinch as a collective whole is akin to saying that Mienfoo is too strong because when combined with Timburr, the combined presence of fighters is too much - we need to look at each mon separately.
To be clear, Arena Trap is still a suspect-worthy issue because both abusers are highly centralizing (whether they're moreso than Mienfoo or Vullaby is up for debate). An Arena Trap suspect has been brought up in the context of GastBraPinch, and I think LCWC has made it abundantly clear that GastBra(trap) is every bit as good as we thought it was, which is way too good.
We've been seeing absurd sets like East's Protect/Disable Grimer-Alola because they feel
necessary. Simply put, our metagame is not equipped to handle Pokemon with the level of offensive presence that Abra and, to a lesser extent, Gastly bring. They can, with minimal changes to a single standard set, get around every other Pokemon in the metagame with relative ease. This is fine for a wall-breaker, not for a Pokemon that outspeed the majority of the metagame and has significant potential to sweep. Balance does not have the general safety nets this generation to deal with such powerful offensive Pokemon, and offense is constrained by requiring several threats that can compete with GastBra in speed tiers (which often happens to be GastBra itself, judging by the number of times we've seen GastBra in LCWC), resulting in a metagame where creativity is often seen as more punishing than rewarding.
This is not the first time Abra has been on the fence this generation - late SM Abra webs comes to mind. This is also not the first time Diglett has been up there; FooVullDig has been extremely strong for some time now. But it's worth noting that in every actual suspect since FletchDig back in 2014, Diglett's immediate role was negligible - TorchDig, FloonDig and so on were fairly average standalone cores that weren't strong enough to play a large factor in their bannings, and Diglett's role was as a heavy metagame influencer only.
In the case of GastBraPinch, the trapper is only so powerful because GastBra has so few checks (and no counters) to begin with. Trappers aren't without their drawbacks; Trapinch suffers from being difficult to reuse in a single game, and the more reliable Diglett sets (Scarf, Eviolite as opposed to Z, LO) are basically non-threats on their own and have been treated as gimmicks in the past. In addition, they're pretty poor in 1v1 scenarios, and mostly rely on revenge-killing to do their work; often, they will be looking to trap Scarfed users or opposing trappers who are locked into the wrong move instead of healthy mons. These drawbacks are serious enough so that most other cores involving them, such as Ponyta-Trapinch and, in the current iteration of the metagame, even Mienfoo-Diglett, are seen as fairly balanced. Gastly and Abra are unusual for sweeping capable abusers in that each have a very strong chance against winning against their checks, to a degree where they not only lack counters, but even hard checks. Abra can hit its trappers with Counter, while both Gastly and Abra can KO Steel-types with HP Fire, for example. As a result, we're mainly left with revenge-killers to deal with them - to be "safe" against GastBra, you'll usually have to run a Pursuit trapper, Diglett, and/or multiple Scarf Pokemon. Against the likes of these, Trapinch and Scarf / Eviolite Diglett have no issues.
As the more immediately over the top of Gastly and Abra, both in terms of raw stats and in how it lacks chip-related counterplay, I would argue that Abra should be seen as the main culprit in the core.
To drive home how GastBra is too strong and how Abra is a problematic element, I've gone through each replay where the core is used and wins (currently 13 wins to 6 losses, excluding the 3 GastBra mirrors). You'll notice that Abra plays a large role in almost all of them compared to trappers. (Abra, Trapinch, Diglett, and Gastly individually sit at 24-11, 18-12, 14-15, and 16-8, respectively.) It's kind of repetitive so feel free to skip it if you already get the point.
https://replay.pokemonshowdown.com/gen7lc-848022235 - ht vs tharam
HT does not use a trapper at all. Once hazards are down to bring Vullaby into range, Gastly and Abra are able to break through ThaRam's team with minimal effort. Abra ultimately cleaned after Gastly was taken out. This is a clear example of how slower teams tend to suffer, as they generally have a rougher time fitting in the multiple trappers required to consistently deal with GastBra.
https://replay.pokemonshowdown.com/smogtours-gen7lc-420068 - spl4sh vs zorodark
Trapinch removes a weakened Ponyta, which could otherwise force a tie vs Abra. ZoroDark would have had other ways to do this, though - if he had sent Vullaby in instead of sacking Pawniard, he could get rocks up with his Pawniard later for the same effect. The rest of Spl4sh's team would have a lot of trouble dealing with assaults from both Gastly and Abra at that point.
https://replay.pokemonshowdown.com/gen7lc-847051635 - lily vs lighthouses
GastBra + Mudbray (no trapper) wins vs GastBra + Trapinch. Lily didn't really have an answer for both Abra and multiple Scarf users.
https://replay.pokemonshowdown.com/smogtours-gen7lc-420076 - tdk vs cs
This game is kind of telling because TDK brought the ultimate one-mon GastBraPinch answer in Disable Grimer, and the slow balance team was still broken through. Chip from Rock Slide was enough so that Grimer could only take out one of Gastly and Abra, allowing the other free reign against a slow team (outside of a weak Staryu) that could not revenge-kill them like TDK's. GastBra has effectively rendered balance almost entirely unviable where it was niche but at least usable in previous metas.
https://replay.pokemonshowdown.com/gen7lc-834012625 - osh vs fran
This is the game that
tcr linked earlier. This is entirely a Gastly sweep with a timely Shadow Ball crit - neither Trapinch nor Abra is brought onto the field even once. Even if Gastly didn't crit, though, it seems clear that Abra wouldn't have had much trouble cleaning up.
https://replay.pokemonshowdown.com/gen7lc-843655573 - lily vs xray
Abra forces xray's Diglett in, Trapinch removes it and does nothing else, and Gastly cleans for free.
https://replay.pokemonshowdown.com/smogtours-gen7lc-410317 - ggggd vs toadow
Here is another game where a trapper is forgone, and GastBra is just used alongside hazards. In this case, Gastly doesn't need to come in. Once rocks are up and Spritzee is weakened, Abra has no trouble sweeping.
https://replay.pokemonshowdown.com/gen7lc-842475083 - ren-chon vs mao
Mao uses a pretty weird offense team that relies on Wynaut for Abra. Though he does have the means to revenge-kill Abra and Gastly individually, mao doesn't really have a good shot at winning the momentum war.
https://replay.pokemonshowdown.com/gen7lc-842483070 - lighthouses vs wabane
Another example of a balance team that, on paper, should be solid against GastBra, falling all the same. Diglett is included for Abra and Chespin is included for Gastly, but Diglett has to be blown on Timburr to be countertrapped by Trapinch, allowing Gastly to dismantle the rest of Wabane's team.
https://replay.pokemonshowdown.com/gen7lc-838348896 - kingler vs xizaaa
This is the picture-perfect GastBraPinch game. Gastly breaks a bulky Pokemon (Timburr) on xizaaa's team; Trapinch gets rid of a single weakened Grimer; and then Abra spends four turns cleaning up the remaining four Pokemon.
https://replay.pokemonshowdown.com/gen7lc-836471371 - trash vs tazzie
The same (trapper-less) hazards + GastBra team that has appeared in a few other games takes the win with an end-game Gastly clean.
https://replay.pokemonshowdown.com/smogtours-gen7lc-407114 - luthier vs xray
A different trapper-less hazards + GastBra team uses GastBra to break for Carvanha to clean. This is pretty typical HO and doesn't come off as GastBra being an issue.
https://replay.pokemonshowdown.com/smogtours-gen7lc-407578 - ggggd vs tdk
This would be a good game to showcase how even just Abra + trapper can be quite potent. After hazards are put up and Trapinch removes Magnemite, nothing on tdk's team can stop Abra.
https://replay.pokemonshowdown.com/gen7lc-832783226 - lily vs trash
A GastBra mirror where trash gets 6-0ed by LO Abra
https://replay.pokemonshowdown.com/gen7lc-829807365 - cs vs jake
Diglett is taken out early, and Pawniard is forced into play (by Chespin) so that Trapinch can trap it. After that, Abra cleans house.
https://replay.pokemonshowdown.com/smogtours-gen7lc-406498
A GastBra mirror where GastBra + Dig beats trapperless GastBra. Diglett plays a pretty pivotal role in revenge-killing both Abra and Magnemite. so that lighthouse's Abra can sweep.
Diglett and Trapinch are both very strong, and both could be deserving of future suspects, but I'm confident that Abra is far and away LC's most pressing issue right now.
Baton Pass is something else I feel we should be looking at, but I'm not sure how we're going to suspect that since it's mostly a policy question.
