(Obviously, "banning it" is not so clear-cut. Are you going to force Instagram to require that everyone uploads an ID to verify their age?)
There is no such thing as "leave it alone for over 18s".
If you institute strict age requirements, those have to be enforced for every user account. So yes, that means every site you use would require you to go through something like a KYC process.
Privacy on the internet would be gone. You can't enforce age restrictions if you allow users to remain private.
I know it's a controversial topic but the argument that age verification and auth services must remove privacy and inevitably leads to mass surveillance is a false narrative which I find kind of funny as if anybody should know these technical realities it's users on hn.