For reasons I've forgotten, I had Battlefield 1918 (a mod for Battlefield 1942) listed as a dead sim, but it seems that it's still active! Version 3.0 was released just this past July.
I know it's not a flight sim per se, but it does include flyable WWI aircraft, and that's a good enough reason for me to talk about it here :)
Most of the community speaks German, which is unfortunate for us English-speakers. But no worries, there's an English version of the webpage available.
Has anyone tried this? I've only seen screenshots and watched videos, but I always enjoyed my time playing BF1942. Will have to give this a try on my journey through the history of WWI flight games.