I have no idea about this, but as of now anyone can register.

For reference, CSAM is Belgium's government portal and a system of login, as far as I know, so I assume it would be used to check if someone is a minor at the time of registration

Child safety is important but implementing this would kind of defeat the purpose of a privacy focused app.

I agree that just gatekeeping children and therefore verifying everyone with a government platform, but then it raises the question on how to improve child safety on an app that's self-hostable with not even user IDs to identify the users?

I don't have an answer to that, but I don't like just saying "This solution is bad", I always try to add "So try this instead, because XXX"