The problem is, that maintaining a stable API seems like not being that easy. Especially in open source projects where most of the people spend their free time

The problem is, that maintaining a stable API seems like not being that easy. Especially in open source projects where most of the people spend their free time and it is easy to make a tiny mistake to break an API. You asked for specific projects. I can mostly speak for the JavaScript community where React seems to be the best example. They even try to maintain compatibility between two major versions, and give good deprecation warnings. Other big projects like lodash, webpack and RxJS seem to handle it good as well. As I mentioned, React handles the migration path really well. Other projects do not provide such an easy migration between major versions...