I think they are more common than you are thinking. I am familiar with several, even going back to the svn and cvs era, all of which predated the whole formalized-and-named CI/CD thing. In my experience, we called this model submit-to-commit, and depending on the specific manifestation, worked with diffs or branches. I'm talking 1990s.

The fancy bits in this implementation from the paper are interesting but the model itself is not that unusual.