The difference is that pre-systemd, there is no safe way to guarantee that all processes are reaped on logout if some of the processes are written maliciously t

The difference is that pre-systemd, there is no safe way to guarantee that all processes are reaped on logout if some of the processes are written maliciously to try and avoid being reaped. And this is a red herring. If the process is malicious, it can just as well use this systemd method to keep from being killed and continue to do its dastardly deeds in the background. Logically, this "feature" can not be about controlling purposely malicious programs, because there's no way to determine if a process is malicious or not just by asking it or by having it assert that it is not. The only reason this exists is to paper over bugs in either the way these daemons are spawned (ssh-agent and pgp-agent, for example, work best when they appear in the execution inheritance hierarchy, rather than as separately spawned programs that run as siblings) or in the way they detect that a session has ended.