Some random foolishly submitted a systemd patch and it got rejected after some back and forth. I think there’s not much more to it so far.

I read in another thread that it was merged

Yeah there was some back and forth. It got approved by someone from Microsoft then unapproved by Lennart Poettering. Basically temporary kerfluffle. There’s a writeup from yesterday, I think this: itsfoss.com/news/systemd-age-verification/

There’s now an unpopulated field to hold a birthdate but it’s not being used by anything. I guess that is sub-optimal though. This kind of thing if it’s to exist at all should be on individual user accounts, not anything system-wide.