May I recommend d's scheme? I personally find it works quite well. Essentially, breaking features are introduced as a 'preview' that you have to opt into with a compiler flag. After a period of some time, it is changed so that the old and new behaviours can co-exist, but a deprecation warning is issued where the old behaviour is found. After some more time, the warning is changed to an error, but with an option flag to change it back to a deprecation. Then, finally, it is removed completely for the compiler.

This makes the maintainers' job a bit harder, but not by a lot, and as a user I find it a nice medium of stability and progress.