Determined sites could already easily do this. Just detect the patterns used. I agree it's a useless change though.
heave_balks_0g@icloud.com
It shouldn't matter for the sign in with apple because sites are already expressly supporting that.
Email aliasing is hard because you want privacy from a herd of users, but then you're locked into that ecosystem versus a domain you control has no herd, but the upside is no lock-in.