Being the naturally curious type I decided to check OBD squadron availability in-game vs history. Didn’t look at everything (it’s not raining that hard) but picked one aircraft - the Armstrong FK8.
According to the mighty internet the FK8 was used by five squadrons on the Western Front: 2, 8, 10, 35, and 82.
2, 8, 10 are in the game. 35 and 82 are not. The units that have the FK8 acquire them at different times along with the typical High Rank, Middle Rank, Low Rank determining who gets one in the Squadron.
Now, as long as the Squadron has any other aircraft assigned to it besides the FK8 you can join it but you’ll be flying a BE or RE. Once it is totally equipped with the FK8 you cannot, logically enough, as it’s not in the game.
So what happens to these squadrons in the Campaign once it becomes fully equipped with the FK8? I must assume they disappear and no longer assigned to campaign flights.
This is probably flamingly obvious to most but I had completely forgotten about it.
The answer: More aircraft OBD!
EDIT: Sources vary a little but the RAF in the second half of 1918 had 99 Squadrons in France. WOFF has 76 available.