The problem is that those APIs are not well documented, so reading the convolute source code may be the only documentation in some cases.

Perhaps there is some documentation, but it is well hidden.

Just these days, after being hit by the fact that the Xorg server has become dependent on systemd, I have begun to search for what elogind is really doing to simulate the login services of systemd. I have not found any easy way to discover that, except by reading the source code, which is not simple at all.

I would not care if GNOME or any other package would add systemd dependencies, but these were accompanied by a document describing precisely the protocols or APIs they use for accessing systemd services, so that it would be easy to write alternative implementations.

The reality is that no such documentation is provided, so the only way to avoid systemd is to become an expert in its internals. This is why I hate when such new dependencies are added.