Off the top of my head...
A lot of heavy sand builds rely on Burn+ Spikes+ Knock to deal with Ferro, going back to Gen 1 mechanics will make it live that much longer and can only be a nerf to these kinds of sand
Amoonguss relies heavily on both Spore and Stun Spore, now that Ferrothorn checks it and Latios isn't a sitting duck when paralyzed Amoonguss' career is over
Specs users in Latios and Torn will probably stop having Sleep Talk, Jirachi is nerfed as well since it can't hax its way through Magnezone anymore aside from not decommissioning stuff with para/ letting in Rotom-W for free
My prediction is these changes will greatly shift the metagame towards offense
A lot of heavy sand builds rely on Burn+ Spikes+ Knock to deal with Ferro, going back to Gen 1 mechanics will make it live that much longer and can only be a nerf to these kinds of sand
Amoonguss relies heavily on both Spore and Stun Spore, now that Ferrothorn checks it and Latios isn't a sitting duck when paralyzed Amoonguss' career is over
Specs users in Latios and Torn will probably stop having Sleep Talk, Jirachi is nerfed as well since it can't hax its way through Magnezone anymore aside from not decommissioning stuff with para/ letting in Rotom-W for free
My prediction is these changes will greatly shift the metagame towards offense