I am aware of the pages you linked to and have on a few occasions tried to look over things. Having read quite a few rather large standers and APIs my impressio

I am aware of the pages you linked to and have on a few occasions tried to look over things. Having read quite a few rather large standers and APIs my impression the "Portable Interfaces" is rather dim. They are disjoint and unversioned notes not presentable portable interface standards. There are also various internal protocols and interfaces which are not documented from what I have gathered. In short SystemD is an implementation. And as they say in there own documents there implementation is and never will be a portable implementation. The "Interface Stability Promise" is referring to the unit configuration file format and the command line interface. This is not stability of the implementation c interfaces. If you want to see a proper portable specification take a look at the Wayland Protocol Specification. There or perhaps POSIX.