But the point is not to catch every possible observable change, as you say, any of those could break someone's build or change runtime behaviour in a non-desira

But the point is not to catch every possible observable change, as you say, any of those could break someone's build or change runtime behaviour in a non-desirable way. The point is to catch changes that are part of the library's public API. That's something the author can be reasonably expected to evolve in a controlled fashion. If you care about the internals, the onus in on you.