The thing that people usually complain about is systemd forcibly setting its process manager at pid=1. I.e. the thing "discussed" in https://github.com/systemd/systemd/issues/12843
There is a secondary feature to run per-user managers, though I'm unsure whether it does run doesn't run without systemd PID1. Though it might only rely on logind.
Makes no sense to me why the service manager part would require running as PID 1. The maintainer just says this:
> PID 1 is very different from other processes, and we rely on that.
He doesn't really elaborate on the matter though.
Every time this topic comes up I end up searching for those so called PID 1 differences. I come up short every time aside from the two things I mentioned above. Is this information buried deep somewhere?
Just asked ChatGPT about PID 1 differences. It gave me the aforementioned two differences, completely dismissed Linux's prctl child subreaper feature "because PID 1 often assumes this role in practice" as well as some total bullshit about process group leaders and regular processes not being special enough to interact with the kernel which is just absolute nonsense.
So I really have no idea what it is about PID 1 that systemd is supposedly relying on that makes it impossible to split off the service manager from it. Everything I have read up until now suggests that it is not required, especially on Linux where you have even more control and it's not like systemd is shy about using Linux exclusive features.