+1 I just stumbled upon this thread while literally googling "How to revert PR back to draft". This would be so useful.
In addition to the use cases already raised here, which affect me as much as everyone else. This feature is also important because Draft PR's don't have the "This branch is out-of-date with the base branch" comment and the "Update branch" button, so the easiest way to bring a long-lived Draft PR back up to date with master, is to mark it as "Ready for review" to get that button back. But since "Ready to review" is irreversible, if your PR isn't actually ready, you either need to add "[WIP]", or you need to update your branch the old-school way of pulling the branch down, merging with master locally, and then re-pushing it.
If "Ready to review" was reversible, we could mark a PR as ready to review, click "Update Branch", and then switch it back to a Draft PR, hopefully before anyone has the chance to merge it in the meantime.
(Of course, the better solution would be to have the "Update branch" button on Draft PRs as well)
Without this feature, Draft PRs is just an incomplete feature, several reasons mentioned above. Pretty sure I made a private request almost as soon as I started using (then stopped using) draft PRs.
If I cant switch status back the draft - I actually don't even want to know that draft PRs existz.