I think the key difference here is to consider toleration vs adoption. Old code is able to tolerate the changes and still work in new ecosystems. There is still work on maintainers if they want to actually adopt the features themselves. Allowing these two concepts to work together is what allows iteratively updating the world, rather than requiring big bang introduction of features.

As for validating your software, the answer is the same as its always been… tests, tests and more tests.