Everything that a maintainer would need to prove to themselves to merge it can be codified in a pipeline.

Or some kind of protocol for building those things in the MR so that any new behavior explicitly demonstrates the new states and transitions.

This is hard if the new MR introduces a completely different paradigm outside the mental model of the reviewer and maintainer. Might be better off completely forking the project and running it in parallel aka taking on the maintainer duties if they feel so inclined to completely change things