>But my main concern with this is applications like Firefox will eventually require this systemd age specific field and a standard systemd function to call. That means this age field will need to be populated and this locking out the *BSDs and non-systemd Linux.

The risk is real, and the solution is to move away from systemd now, not wait until it's too late. Whatever conveniences it brings over other init systems are certainly not enough to justify giving up online anonymity forever.

> Whatever conveniences it brings over other init systems

You see people rave about the greatness of systemd, then they turn to deploy their applications using Docker and some s6 config.