…appropriate for systemd-nspawn with its goal of running a full init system inside a container environment, but inappropriate for system services, that should be integrated into the host even if they run at a lower security level, with sandboxing applied.
So, what changed? We realized over time that the logic systemd-nspawn implements is to a large degree the same as the one service management implements, and we basically have two implementations of some non-trivial code in place.
Moreover, …
… with tools such as systemd-run/run0 much of the functionality of systemd-nspawn ended up being available for service management too (such as the pty interactivity), with only some parts remaining that only nspawn could do.
So in order to streamline things and simplify our codebase, we figured it might be nicer to eventually merge the two, or more specifically: have a full-blown implementation as part of service management, and then…