It would be a delicious irony if Twitter had intervened to strip Erdogan of this special status. Too bad it never happened.
You can, in fact, have your blue check mark revoked if you violate Twitter's rules or terms of service. Under those conditions, you can't impersonate somebody else, post people's sensitive personal information or facilitate illegal activity, among other things.
It's not clear whether banning your citizens from using Twitter constitutes illegal activity. But in any case, Erdogan wasn't de-verified, because he was never verified by Twitter in the first place. Here's his account today:
And here's a tweet from his account as of last September:
Notice that there's no blue check mark on either image. Besides, notes University of North Carolina assistant professor Zeynep Tufekci, Erdogan's account appears to be state-run: It doesn't contain any tweets written by Erdogan himself. Which, incidentally, probably helps explain why Erdogan found it so easy to quit the service.