As the person behind a lot of the SVE work mentioned in Nicolai's response, I can say that the lack of a documented policy on this has been incredibly frustrating for me. I've been told by code reviewers to remove parts of patches aimed at maintaining stability. I've been told weeks after the fact to add such changes. I've had people put the breaks on my work because they did not read the RFC submitted to the mailing lists months prior. And through all of this I had no ground to stand on to push back on any of it. I'm fairly new to this project, so I have little personal clout with which to assert myself, and the policy is entirely oral tradition.
For the time being, I will not take a position whether or not "flag-day" changes are optional, encouraged, discouraged, required, or forbidden. I just ask that we please decide on something, and write it down. If the current practice is not acceptable, then that should be raised in a separate RFC, but we should still write the current practice down.