[kazinator]

If a new feature is completely buggy, but it does not impact any existing features, then that doesn't require any compatibility switch. You just fix the feature and issue an update or new release.

If a new feature is buggy but useful (say, the behavior is wrong in a way that users figure out, and start depending on), then you can have an option to emulate that buggy behavior.

If a new feature causes a regression in existing features, then that is a call you have to make. You have a situation in which something worked up to version K. Then was broken between K+1 and L, either not working at all or working differently. Then as of L+1, it was discovered, fixed and works again as before.

If it was completely broken, then you just fix it and that's that. If it was broken in ways that left it useful, such that users may have come to depend on the altered behavior, then just subject it to compatibility. Emulate that behavior if users request compatibility with a version between K+1 and L. If they request compatibility with K or less, or L+1 and higher, enable the current fixed behavior.