Expect this to be posted in the Gliscor Suspect Thread.
In my honest opinion, Gliscor isn't overpowered. Nearly all playstyles have very easy ways to check it and annoy it.
Hyper-offense has numerous mons to chip it, trade with it, then wreck it quickly, such as Iron Moth, Roaring Moon, Iron Valiant, Ogerpon-Wellspring, etc. Weather teams also have numerous ways to handle it, despite them losing popularity as of late. Webs isn't too bothered either, as Meteor Beam Glimmora outright threatens to OHKO it and the main Webs setter as of late being Surf Araquanid (thanks Duckular).
Offense has numerous Pokémon such as Primarina, Taunt Landorus-T, and Ogerpon-Wellspring that all work together to handle it regardless of what set it is and what it decides to invest it.
Even balances and bulky offenses have numerous ways to defeat it, such as Weavile, Boots Kyurem (who's now back!), the aforementioned Primarina, Ice Spinner Great Tusk, CB Rillaboom, IronPress Corv, Hex Dragapult, Taunt Lando-T etc. Not that any of these Pokémon are permanent, solid answers on their own, but they're all good balance/BO mons that can be slotted together to create a solid plan against Gliscor.
Even bulkier teams have ways to tech for Gliscor. Clefable eternally has an empty slot besides Moonblast, Rocks, and Moonlight, and Ice Beam works perfectly to snipe not only Gliscor, but Lando-T, Garchomp, Dragonite, and the like. IronPress Corv is another solid answer that can be found on bulkier teams. Physically defensive Slowking-Galar is also okay with running Ice Beam as its third slot, and while it's nowhere near a solid answer, it can help. For styles such as Bulky Hazard Stack, Weavile and Darkrai are fantastic anyways as Knock Off users, and they both carry nukes to hit Scor with.
Finally, hard stall and semistall. Semistall often carries Pokémon such as Weavile, Hoopa-Unbound, Meowscarada, or any breakers that punish enemy passive teams, all of which destroy Gliscor as a result of their breaking prowess. Even hard stall has ways to beat quite a lot of Gliscors, from Ice Beam Clefable (although admittedly hard to fit), Avalanche Dondozo, IronPress Corv, Tera Ice Hydrapple (who isn't super common but exists), and Weezing-Galar, who all feature usage on stall. While yes, SD Gliscor's matchup into stall is certainly unfortunate at times, stall can and should have answers to at least partially mitigate the issue.
But the question is, what if Gliscor decides to Terastallize? Well, Tera is generally considered worth a Pokémon or even more, and while Gliscor is a fantastic abuser, it's still a weakness. Firstly, it's still incredibly weak to Kyurem, Meowscarada, and Ogerpon-Wellspring with Tera Water, and terastallizing to it will likely do nothing outside buy you a few turns against those 3. Tera Dragon is another common one, but it creates massive new weaknesses in the form of Fairies and fast Dragons such as Raging Bolt, Dragapult, and Iron Valiant. Finally, Tera Normal. Tera Normal is fantastic on some sets, however it often leads to you losing to IronPress Pokémon, Close Combat users, and gives you very few resistances to lean back on.
Gliscor is still a fantastic Tera user, I won't deny it. But every time it terastallizes, it gains new weaknesses to extremely common Pokémon that can be easily slotted on many playstyles, and also gives up its ability to be nearly hazard-immune, making it take hazard damage all over again if it's forced to switch out by a Pokémon that threatens its new type.
While I do think Gliscor is an amazing Pokémon by all means, and deserves heavy consideration, planning, and prep on all playstyles, I don't think it's broken or banworthy specifically because every playstyle except arguably hard stall (who almost always use Gliscor anyways) can tech for it in numerous easy, slottable, and varied ways to answer not only its base typing, but its many teras as well.