I blame companies (including discord) for collecting as much information as they can instead of as little as possible. More data collected -> more data that will eventually get sold / leaked / hacked.

Don't governments require them to chech people's IDs to make sure they aren't kids?

Do they also require permanently storing the document instead of just the check result?

Oficially, no, unoficially, yes.

It depends on the implementation. The EU's European Digital Identity Wallet will allow users to prove that they are over 18 without sharing any other personal information.

Anonymous means you can pay someone $2 to use theirs.

Surely that's solved easily by ensuring a 1:1 association between the proof of age and account?