[bryanlarsen]

It's 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.

But the original question is beside the point -- systemd-appd will provide an API that can be implemented by others. I doubt the questioner actually cares much about how much of systemd systemd-appd pulls in, they want none of it.