As of today it seems no longer possible to self-approve PRs.
Event worst, a pullrequest can not have any Status that the author can set in order to mark whether the PR is open to review or what....
It is such a basic feature. We used self-approval in our workflow to indicate to the team "you now review the PR". A missing self-approval was indicating "I'm still working on this one".
Why would you remove such a basic feature? Every other GIT tool has a Status attached to a Merge Request.
Update: the button for self-approval is present for some PRs. But I cannot see a clear pattern.