Time to become a necromancer and revive this thread with some noms.
Addition
Hatterene UR -> C Being able to stop Ferro completely is pretty impressive alongside Mudsdale, Seismitoad, and Clefable to some extent, and considering it has Nuzzle, switching can be annoying as well. Moonlight halved by weather is annoying though. But it can also Teleport on things it walls, oh, also trick Room.
Tangrowth UR -> C It has a small niche in stall teams.
Durant UR -> C It's very strong and can slam teams without Steel, and even Corviknight with Rock after one U-turn... 80% of the time.
Metagross UR -> C+ I tried Demon Metagross and gotta say, it can get out of control very quickly. One advantage it has over Mew is Steel type, so you can't beat it with toxic.
Umbreon UR -> C I had mixed results with Umbreon. On one hand, its bulk is definitely great, with Psychic immunity and Ghost resist coming in handy. Access to Spin also gives it an important niche over Ttar. However, most offensive mons being dual type make its a letdown at times, tough against Torn-T, Nido and Rotom it is better still.
Bewear UR -> C+/B- This thing is really fat. Effortlessly taking hits from Ttar, Zeraora and might even live 2 Brave Birds from Aero, its biggest weakness remains Sunsteel Strike. It can also run No Retreat to some extent, in addition to Spin and Recover.
Sigilyph UR -> C What makes this viable for me is not Magic Guard, but Specs Tinted Lens. Being something that can slap both AV Slowking and Pex is no joke. Obviously can't be higher since it can't 2HKO SpDef Ttar (Can still Trick though)
https://replay.pokemonshowdown.com/gen8stabmons-1593026713-ggay56pgyn2e8wxo96q14wqxgzq2lvipw
Diancie UR -> C This mon was the reason I believed Thunderous Kick wasn't broken back then, nowadays it can still deal with Fire Lash. It's also a rare Pokemon to resist Mence's dual STAB and doesn't fear the random Superpower or U-turn from Torn-T. Most of the time Ttar is preferred still though.
Stakataka UR -> C: Trick Room
Drampa UR -> C+: All Trick Room should have This, Specs hits so hard, and Cloud Nine makes sure Ttar won't be saved by Sand SpD boost.
Talonflame UR -> C: Besides Ttar, Dragon Ascent is a pretty fun option for offense I saw in Swiss
Doublade UR -> C: This is very underexplored. SD hits hard with Poltergeist + Iron Tail.
Jolteon UR -> C: It's like special Zeraora, except thanks to Boomburst and Encore, it can force Thundurus out. Weaker Boomburst compared to Heliolisk, but being faster and bulkier gives it a niche.
UR -> C Grouping these two up since they are similar: Fast Web setters.
Luxray UR -> C I swept pannu with this, I will let you judge whether that makes it good.
Poliwrath: UR -> C Being a rain sweeper with Belly Drum can be deadly I guess
Stoutland UR -> C+ Benefits from sand being strong atm, I find this a decent option with SD and Memory Multi Attack to allow Exca/Dracozolt to sweep (Usually Fire Memory for Ferro and Corvi).
Change
Pelipper C -> C+ I think rain is pretty decent all things considered, making Pelipper better.
Victini B+ -> A- Being able to bluff a normal set to freely get Screens and then click Final Gambit is huge, and it can even fit Encore as well. Would recommend trying.
Volcarona B- -> B A hybrid offensive set with Quiver and Web does me wonders. One QD and you force out all Defoggers/Spinners, can even run Mind Blown if you want a surprise suicide Web lead.
Arctozolt C -> C+ It clearly doesn't belong in the same tier as other mons. While slow and relies on Hail being up, once it gets a sub up it can be a pain to get rid of, since it can pack Low Kick for Ferro as well.
Chansey C -> C+ Transform Chansey is pretty obnoxious to take down.
https://replay.pokemonshowdown.com/gen8stabmons-1609393125-iapalt49zk7zslu94mxb6vtvc58961kpw
Cloyster B -> B+ Might even be A-, this thing is VERY scary
Porygon2 C -> C+ Specs Boomburst hurts, I mean it. Struggles against Tyranitar but you can add support for it. Like BJ vs Dugza
Steelix B- -> B Might just be me, but I really like having a solid Aero answer, considering it beats Spin Clef with Anchor, as well as Ttar and Zeraora
Hydreigon B+ -> B Latios just gives it too much competition imo, Hydreigon can't actually break Toxapex, which is a massive drawback whenever I see it. Mostly relies on Switcheroo to cripple Pex and that's it.
Thundurus-Therian A -> A- I don't think meta is kind to it atm sadly, with offensive mons all being faster and it keeps having to predict correctly, otherwise it won't make much, if any, progress
Reuniclus B+ -> B Not gonna lie, I almost forgot this exists. Feels like it can be deadweight in too many games and not consistent.
Changes that I'm not sure on:
Slowbro-Galar UR -> C Belly Drum Quick Draw Quick Claw can sweep, probably
Gastrodon B- -> C+ I have no idea what this has over Seis.
Barbaracle UR -> C Like Kingdra, but Shell Smash > Dragon Dance and can actually break Ferro
Regigigas UR -> C I swear this is not bad, it gets Recover and SD now!
I think I might edit some replays in later