By all intents and purposes, it is strictly worse than an ability that has zero reason to be banned.
Fairy Aura and Dark Aura are strictly worse than Adaptability in singles, but manage to be stronger abilities in practice. Same for Arena Trap vs Shadow Tag in gen 5 -- the former was banned, but Shadow Tag is still legal. This has never been a problem for tiering policy, because distribution is an aspect of the ability, and being given to better Pokemon is something that makes an ability better. We ban things because of the actual problems they present in this metagame, not because of the problems they might theoretically pose if they had better distribution. It makes no sense to insist that OU uses Balanced Hackmons tiering policy, because we aren't playing Balanced Hackmons.
(For another, admittedly very facetious example: we don't ban Primordial Sea in tiers where Drizzle is banned "just for consistency" if nothing in the tier actually gets it. We accept that banning Primal Kyogre in UU accomplishes the same thing in practice.)
Shadow Tag is entirely different. Your example is also a straw man. Obviously the ability isn't broken on a Pokemon with a 30 BST and only knows Splash. That's an ABSURD counterargument. Short of an ability that KOs the opponent upon entry, nothing could make such a Pokemon broken. Yet may I remind you that Wynaut, a Pokemon with 93/48/48 bulk and at the time literally only four moves, was banned to Ubers before Eviolite even existed.
So what you're saying is that Shadow Tag is banworthy because it's broken on everything that gets it. This supports my point just as much as it counters it. I'll admit that the existence of a 30BST Pokemon with STag wouldn't mean we had to ban every STag Pokemon because a non-broken instance of the ability existed, though.
I can appreciate the difference between an ability being banned for being uncompetitive vs being banned for being overpowered, but I'm not convinced that it makes a difference in terms of how we should handle it. Generally, the reason we ban uncompetitive abilities outright and not overpowered ones is that their orthogonal angles of attack mean they produce similar metagame problems on their weaker users when the stronger users are removed. Gothita STag and Arena Trap Diglett are good examples of this.
I think your stance is that uncompetitive abilities are disallowable in isolation, but this is not consistent with how we have handled those abilities in the past. Heck, even in this generation, we didn't re-ban Moody just because it is conceptually uncompetitive. We banned it once it started producing regularly good results in high level play with the right roll. In the case of both Moody and STag, we saw the actual material effect on play before making a difference -- something which inevitably takes distribution into account to a degree, because the abilities will obviously only be used on things that can legally get them. In short, I think we have handled uncompetitive abilities differently because they tend to be problematic on every user, not because of a core difference in tiering philosophy.
Gorilla Tactics is just good and fundamentally sound, but too good when paired with every other good thing Galar Darmanitan has.
Conversely, Galarian Darmanitan is good and fundamentally sound, but too good when paired with Gorilla Tactics.
The difference is that Gorilla Tactics is, by current legality, not capable of not coexisting with Darmanitan-G. Darmanitan, however, is capable of not coexisting with Gorilla Tactics, and is probably not overpowered when it does.
the whole inclusivity argument seems to be popular, but maximum inclusion is not the goal of smogon's tiering. instead, our goal is to ban the root of the problem.
banning protean instead of greninja in oras, for example, would promote inclusivity. greninja lacking protean objectively adds more to the metagame than protean kecleon. you could argue for similar cases such as banning sheer force instead of lando-i, banning flyinium z in uu to let in dnite/mence/gyara, and others. sure, there's collateral, but the point is that the metagame gains more viable things than it loses.
I don't think these are good comparisons because none of these things are signature abilities/items. There is a choice between two sets of collateral damage for keeping the ban simple in your examples, and that's not a factor here. There would be absolutely zero difference, other than allowing Zen Mode Darmanitan, if we banned Gorilla Tactics, and I think that makes Gorilla Tactics something of a special case.
The fact that there are zero non-broken instances of Gorilla Tactics is important. Sure, it's that way because only one thing gets it, rather than because it would be broken if other things got it, but it's not at all self-evident to me that tiering philosophy should care *why* it's broken on everything that gets it.
I would also argue that Speed Boost is the root of the problem, rather than Blaziken -- it's inherently a very good ability, whereas Blaziken is a fairly mediocre Pokemon in isolation. Feel free to correct me here, but I think we chose to ban Blaziken because tiering philosophy is to prefer banning Pokemon in cases where conflict between two sets of collateral damage exists. So I think the lack of any collateral damage is important here: it's more like the Mewnium Z ban in UU last gen, which was banned for power level reasons and was banned instead of Mew because the non-broken element could be preserved by a simple ban with zero collateral.
I will accept, in the interests of fairness, that there might be precedent for not banning broken signature elements and banning the Pokemon instead. Marshadow springs to mind, but I don't know if Spectral Thief would have been considered for a ban if it wasn't for Smeargle existing. It probably would have been seen as a pet nerf, even though Spectral Thief offers unique functionality. I feel like moves and abilities/items ought to be treated differently here, but I readily accept that there are grounds to disagree.
Edit: another factor that just occurred to me. We tier certain things differently in order to tier different forms separately, such as banning Power Construct or certain Mega Stones. Is it worth doing the same to *allow* a specific form? Should we be trying to preserve Zen Mode Darmanitan-G as a different form, even if we would not otherwise try to preserve an alternative ability? Or does the fact that the form change is battle-conditional preclude this?