The base version for uselessd is systemd-208, which is the version used in 13.1. Let's try if a direct substitution of the binaries works and watch out for the problems.
Expected result of the project is to have a working package with "Conflicts: systemd" and "Provides: systemd". The goal is not to fix all problems, a stripped down system with uselessd is considered a good achievement. Anything more complicated could build on top of this.
It is well-known that two git commits within a single repo can be independent from each other, by changing separate files to each other, or changing separate parts of the same file(s). Conversely when a commit changes a line, it is "dependent" on not only the commit which last changed that line, but also any commits which were responsible for providing the surrounding lines of context, because without those previous versions of the line and its context, the commit's diff would not cleanly apply.
As with most dependency relationships, these form a directed acyclic graph. Sometimes it is useful to understand the nature of parts of this graph; for example when porting a commit "A" between git branches via git cherry-pick, it can be useful to programmatically determine in advance the minimum number of other dependent commits which would also need to be cherry-picked to provide the context for commit "A" to cleanly apply.