It would be rather simple: "Here is the testsuite for our (public) api. We support the use cases tested, but no more.
If you want your use case covered, please submit a testcase.
If we change an existing test, we increase the major version.
If we add tests, we increase the minor level.
If the test suite remains the same and we only change the code, it's a patch-level increase."
This could be verified automatically, increasing trust.
A function is compatible if it requires no more than it did, and provides no less than it did in the previous version. A library is compatible if it provides no fewer functions, and requires no additional context (transitive dependencies) to use it, and all of its functions are compatible as defined.
When you wish to require more, or provide less, you make a function with a new name.
[1] As linked elsewhere: https://youtu.be/oyLBGkS5ICk?t=1189
But I am left wondering something. How can you easily let people find out if their use case is included in the tests without having them read 20,000 tests.
That's more for HTTP communication between services but I wonder if you could build something similar for libraries.