Is it? I wouldn't be surprised if immunity to nonmagical weapon damage is gone, which IMHO is a good thing. It was always kind of annoying that I didn't feel like I could use wererats at a lower level unless I gave PCs silver or magic weapons. Gargoyles? Like them and they're an interesting monster. Rarely use them as written at levels that match their CR because most PCs won't be able to do anything against them.
Do you have a reason for believing it's entirely gone from new monsters? I don't have an issue if it is, it's always been a bit of a weird property because their CR really changes based on whether or not the group has magic weapons available.
Released 2024 monsters.
For example 2014 quasit:
Damage Resistances Cold, Fire, Lightning; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks
2024 quasit:
Damage Resistances Cold, Fire, Lightning
Edit: And some circumstantial evidence.
Monks attacks upgrade to force damage, all reference to counting as magical gone.
The Heavy Armor Master feat no longer protects against only nonmagical damage. Could be just an upgrade, but with everything else known, unlikely.