This answer assumes it's still 2020 May. It's probably already outdated, so don't take too much advice from it.
Skarmory (and Greninja, Landorus, Heatran, Chansey, Garchomp, Gliscor, Swampert, Diancie, mega Tyranitar, and Azelf) is banned in gen 8 OU.
Spikes Mew is banned in gen 7 OU.
Stealth rock Ferrothorn, Excadrill, Clefable, Jirachi became less common, as you expected.
Spikes Ferrothorn and Accelgor, stealth rock Kommo-O, Tyranitar, Mew, Hippowdon, Seismitoad, Terrakion, Shuckle, and Rhyperior, toxic spikes Toxapex and Accelgor, and sticky web Shuckle became more common for some other reasons.
Talonflame is banned in gen 8 OU.
Centiskorch (and Cinderace) is banned in gen 7 OU. Centiskorch is bad in gen 8 OU even when it has heavy-duty boots.
Heat Rotom, Mandibuzz, and Salazzle became more common, as you expected.
Cloyster, Darmanitan, Ninetales, and Accelgor don't use heavy-duty boots for some other reasons, even though they are threatened by hazards.
Gliscor, Greninja, Landorus, Magearna, Magnezone, mega Scizor, Tapu Koko, Tornadus, Victini, Alomomola, Amoonguss, Gliscor, mega Latias, mega Sableye, Slowbro, Tangrowth, Tapu Fini, mega Tyranitar, and Zapdos are banned in gen 8 OU.
Aegislash, Cinderace, Dragapult, and Corviknight are banned in gen 7 OU.
Ditto, Tyranitar, mow Rotom, heat Rotom, Salazzle, Zeraora, Clefable, Celebi, Seismitoad, Jellicent, Mandibuzz, Sylveon, and Incineroar became better pivots, mostly because gen 7 OU's good pivots got banned.
Hazards became less common overall.