The stupid thought I keep having is what stops someone from patching the functionality out?

Nobody has to patch it out. The OS just doesn’t have to use them.

As long as the OS doesn’t implement age verification, that field in systemd means nothing. Some OS will have to do that - especially commercial distros based in murica. So the change within systemd is largely inconsequential, but people are freaking out about it - most likely because they don’t understand systemd and what it does.

If it means nothing, then why adding it?

Because some OS vendors based in the US might have to comply with the law, if you like it or not. System76 or RedHat will not have a choice unless the laws will get repelled.