Bug
Bug overall seems pretty bad considering Volcarona is hard walled by Heatran at the moment due to its lack of Ground coverage and Buzzwole being countered by Celesteela's Flamethrower or even worse, Air Slash. The Steel vs Bug matchup has caused Centiskorch to get more usage with RestTalk + Fire coverage or even Scorching Sands that fishes for Heatran specifically. Even with Melmetal gone, Bug vs Steel will still be one-sided.
Dark
:incineroar + Knock Off. Hydreigon has always been a nice steelbreaker with Nasty Plot + Flamethrower/Fire Blast as it sets up on passive pokemon like Ferrothorn and Aegislash or even more Pokemon like Celesteela, Corviknight, or Heatran with the support of
Grimmsnarl's Dual Screens. Urshifu can still heavily chip this as you can see with this calc:
+1 252 Atk Urshifu Close Combat vs. 244 HP / 0 Def Melmetal: 432-510 (91.5 - 108%) -- 81.3% chance to OHKO after Stealth Rock , Melmetal is OHKOd after a Bulk Up. Choice Band and Life Orb Urshifu sets can be seen more in use but that was just featuring your standard Black Glasses Urshifu. Incineroar was used at the beginning of Monotype when both Kyurem-Black and Melmetal were here as it has useful Knock Off Utility + Parting Shot to bring in your neighboring abusers/wallbreakers via Hydreigon and Urshifu to improve the matchup. Incineroar also has access to Flare Blitz and Overheat so I feel like its worth exploring once again. With various mentions above, Dark has a good chance against Steel.
Dragon
Dragapult with Choice Specs Fire Blast is nice but since Heatran is here, you have to think out your turns more rather than spam Fire moves vs Steel type. Kommo-o doesn't KO Melmetal sadly but it can potentially improve the Steel matchup or even as a lead with Body Press + Flamethrower. Choice Band Dracovish is 2HKOd by Melmetal's DIHB but with a calc below, you can notice it OHKOs Melmetal after some chip. Hydreigon with Choice Specs Fire Blast + Focus Blast should see some more usage while +2 Garchomp KOs this. Garchomp would probably need something like Fire Fang to hit Ferrothorn + Corviknight/Celesteela effectively though. This matchup on paper seems pretty winnable
252+ SpA Choice Specs Dragapult Fire Blast vs. 244 HP / 0 SpD Melmetal: 466-550 (98.7 - 116.5%) -- guaranteed OHKO after Stealth Rock
+2 Atk Garchomp Earthquake vs. 244 HP / 0 Def Melmetal: 480-566 (101.6 - 119.9%) -- guaranteed OHKO
252+ Atk Choice Band Strong Jaw Dracovish Fishious Rend (170 BP) vs. 244 HP / 0 Def Melmetal: 390-460 (82.6 - 97.4%) -- 25% chance to OHKO after Stealth Rock
Electric
Electric does not seem to have problems with Melmetal on paper especially with Heat Wave Zapdos. Choice Band or Bulk Up Zeraora helps a lot versus Steel althought it may need +3 to KO this. Rotom-Heat is a direct counter for Melmetal but isn't really needed but throwing it out there. Magnezone can still trap Steel-type Pokemon and I doubt Melmetal will have much intentions of spamming Superpower in that matchup. Finishing this off with Focus Blast Pokemon like
patches this argument up.
Fairy
Fairy needs screens up to even have a chance against this matchup. Azumarill with Knock Off is nice for getting rid of Melmetal's item or even statusing this with Primarina forces it to use Facade or a weakened DIHB in combat. Tapu Bulu with SD/Close Combat/High Horsepower/Horn Leech allows it to act as a direct steelbreaker while Clefable has Fire Blast Coverage. The Tapu Bulu sets are kinda everywhere between Choice Scarf, SD Zen Headbutt (lol just run HH) and I feel like the mentioned set above is the best. Fairy is still a little underplayed but starting out with a general squad like Tapu Bulu/Klefki/Clefable/Azumarill should do the trick.
+2 252+ Atk Life Orb Tapu Bulu Close Combat vs. 244 HP / 0 Def Melmetal: 546-645 (115.6 - 136.6%) -- guaranteed OHKO
252+ SpA Life Orb Clefable Fire Blast vs. 244 HP / 0 SpD Melmetal: 393-463 (83.2 - 98%) -- 25% chance to OHKO after Stealth Rock
+1 252+ SpA Life Orb Clefable Flamethrower vs. 244 HP / 0 SpD Melmetal: 481-567 (101.9 - 120.1%) -- guaranteed OHKO
Fighting
I feel like its a number of ways you can chip Melmetal with Fighting but the main thing is + potentially causing Fighting havoc if not played right. LO Urshifu, Scald(after chip) or Focus Blast Keldeo, Thunder Wave/Focus Blast Cobalion, and neighboring teammates like or can do a lot.
Fire
Flying
Tornadus is able to get rid of Melmetal's Choice Band and survive and pivot out with u-turn. Zapdos and Skarmory can come in and recover pretty easily thanks to Roost. Landorus with Gravity helps a lot with Steel or just FB + EP if its not Gravity. Other notable mentions are with Flamethrower + Scorching Sands , with Focus Blast, and with Flamethrower with both defensive and offensive sets.
Ghost
Specs Spectrier can threaten Steel as a whole after some chip on a mon while Blacephalon does the same when Heatran is chipped. Alolan Marowak KOs Melmetal after Stealth Rock with Flare Blitz and isn't completely walled by Heatran. Galarian Corsola just status's this and recovers with Strength Sap. Can even creep Melmetal if you are worried about it with 48 Spe.
Grass
Don't flame me for mentioning Tangrowth lol but I seen Chaitanya use it before and its the first thing that comes to mind. It has useful utility like Knock Off and Sleep Powder which seems to do decently vs Steel on paper. This also supports Zarude on using its steelbreaker set: Bulk Up / Drain Punch / Darkest Lariat since its asleep. Knock Off Pokemon in general like
can get rid of its CB.
Ground
252 SpA Life Orb Sheer Force Landorus Earth Power vs. 244 HP / 0 SpD Melmetal: 647-764 (137 - 161.8%) -- guaranteed OHKO
Ice
Sadly, Melmetal makes Veil look like a thin piece of paper and with Heatran here, Galarian Darmanitan isn't as able to just spam Fire Punch/Flare Blitz. The matchup overall seems to be "get Ferrothorn out so" can sweep.
252 Atk Choice Band Gorilla Tactics Darmanitan-Galar Flare Blitz vs. 244 HP / 0 Def Melmetal: 456-538 (96.6 - 113.9%) -- guaranteed OHKO after Stealth Rock
Normal
With the likes of and is seems harder for Bewear to pose a stronger threat against steel. But Bewear is ok with Bulk Up I feel but Choice Band was used at the beginning of Monotype. Choice Specs Heliolisk is pretty much 50/50s with Focus Blast(if it lands) or Thunderbolt/Dark Pulse for the Aegislash switchin. Obstagoon gets rid of its item while Diggersby has Earthquake and Fire Punch. This matchup seems very tough but will it improve with Melmetal gone? Yes, but Celesteela + Aegislash + Cobalion still poses a threat against Normal.
Poison
Nidoking is the best steelbreaker Poison has and Earth Power OHKOs this. Knock Off Drapion and Toxapex exists and statusing this does not seem like the best route unless you have something like but otherwise Facade is pretty spammable if there if none. This matchup seems pretty 50/50 to me.
Psychic
Nasty Plot Fire Blast Mew was originally used as the steelbreaker and now with SpA attacking Victini, that matchup has improved tremendously. Kudos on Decem for is creative Steelbreaker Victini set with Blue Flare/Scorching/Sub/Glaciate.
252 SpA Victini Blue Flare vs. 244 HP / 0 SpD Melmetal: 504-594 (106.7 - 125.8%) -- guaranteed OHKO
Rock
I seen a few ladder games with Terrakion and Lycanroc Dusk doing a lot versus Melmetal Steel but Rock overall just never seems to have a fair matchup. Perhaps positioning SD LO Terrakion in the right place. The Rock vs Steel mu will improve slightly when Melmetal is gone but critical play will still be required.
Steel
issa mirror
Water
Urshifu, Keldeo, CC Barraskewda, and Dracovish all do a lot vs Steel and everything resists its STAB pretty much