I was wondering just now if an official clan has ever gotten their official status taken away, as in being turned back into an unofficial clan.

And before anyone links me to the dead clan list, no, that isn't not even remotely close to the same thing, so don't do it.

Pretty sure that that's impossible. But sometimes an official clan dies and is revived. When revived, it has to start all over in the clan discussion board with 1 thread instead of a whole board, but they still have the brackets around their clan tag.
A couple of examples are Diamonds and My Litle Ponies, probably some more.

No. Anything that would merit a clan's official-ness to be revoked would be solved by banning the dunce who caused trouble for the clan.