Your membership wasn't revoked.
What you lost was Oesau usergroup access.
The reason was that you're not actively flying IL-2 or participating in Oesau events. Others lost this access too - for example, RivRat, Lowengrin, etc.
If you start flying IL-2 again with the squadron, you'll get it back.
Regardless, you should still be able to post over on those boards with no problems.
Hope that makes more sense.