Could have been deleted for a couple reasons. The UK project is not really on board with the naming standards and uses single categories for its military for all time. It also could have been done to keep all the records in the Royal Flying Corps and not creating duplicates for all records that were pre and post Mar 1918.
Since the RFC was involved in the conflict from its beginning in 1914, there would be a lot of records that would not be included in RAF post Mar 1918, i.e. people whose enlistments were up, medical discharged, KIA, MIA, etc.
I recreated the category.