I don’t even know where this rumour came from.

It’s not a rumor, systemd merged a PR that explicitly said it was to allow handling the new age verification laws. Just because they aren’t actually verifying anything doesn’t mean that they didn’t merge code in direct support of the laws. And why in the world would this even be handled at systemd level anyway?

Ideally, we wouldn’t need to do age verification at all. But if it’s absolutely required, the most privacy-preserving way would be:

System administrator verifies the birthdate of the user, stores it on the user account

Applications can then ask, “is the current user over 18?” And just gets true/false.