Well I'm not here to argue about systemd in general, that's a far larger topic. All I'll say is that from my experience, systemd felt cleaner than the competitors at the time and created a lot of conventions and standards that are taken for granted today.
For example, to bring this back to the original topic, another commenter in this thread mentioned that systemd provides an API for generating randomized machine-ids that flatpak utilizes.
Though flatpak is gaining more dependency on systemd, and I don't think that coupling is necessary, so I've been keeping an eye out in that space.
Edit: researching a bit more on the topic, there's some interesting comments here about how systemd is not as monolithic as people think
flatpak is not depending on systemd as a whole, they're depending on a new component called systemd-appd...it is an API + a spec, just like what you're asking for. But it's implementation first, because any API + spec designed by committee rather than being driven by an implementation universally sucks.
[systemd] not a monolith, which is obviously true because there is no distribution that ships all of it turned on by default. Fedora comes close, but most other distros pick and choose which parts of systemd to use.
systemd-appd will likely depend on some but not all of the rest of the systemd system.