Being such an old game it runs on all-custom code he refers to as "YSFlight Kernel" that don't even support texturing, but the game is extremely lightweight and its flight model is at least bearably realistic. That casts contrast to many commercial games like Ace Combat franchises. It also has good keyboard support.
During 2010s the author followed open source movement and dumped some code on GitHub, but the core value of YSFlight remains its easy and compact nature. It's a worthwhile 20MB on your doomsday gaming console to bring to your designated fallout shelter.
To be clear, YSFlight is partially open-source software.
Here is official repo[0] of its GUI toolkit (OpenGL-based) and various miscellaneous apps based on it.
PolygonCrest (aka `ysgebl`), an official 3D editor for addon making for YSFlight, based on the same toolkit as YSFlight, is fully free & opensource.
[0] https://github.com/captainys/public
[1] https://github.com/captainys/public/tree/master/src/ysgebl
I do remember there being a server plugin of sorts that somehow managed to do things like darken the sky as your aircraft climbed. Can't remember what that was called.
Yep, that was the real problem till YSFlight version ver.2015xxxx, but since ver.2018xxxx it was a little bit fixed, but I'm agree that being open-source it could has a chance to be fixed by community.
Being in actual state, YSFlight is hobby project of single person.
That is why its hard to predict future of this amazing software.
Nothing against closed source though. For example, I enjoy x-plane a lot which while not free (in either sense) of course has a large community providing both free and non free add-ons. This seems more of a game with a focus on combat and less on realism or looks.
An actual free, open source flight simulator that deserves mentioning is Flightgear. I've played with that a couple of times but always end up reverting to x-plane. But flightgear has some nice features and also a nice community. It just doesn't look and feel anywhere near as good as x-plane (which is a tall order because it is very good).
Anyway, I checked out some footage on Youtube. Visually, it has nothing on even Flightgear; which I'm pretty sure probably has some combat mods. Or even any version of that the last few decades or so, actually. It actually reminded me of some dos games I used to play in the early nineties. E.g. Jetfighter II (released 1990) was pretty awesome back in the day. It looks like that but with higher resolution. But e.g. the ground is a featureless green blob; just like in Jetfighter II, and clouds are white polygons hanging in the sky, etc. And Jetfighter II had a HUD that was about as feature rich as the one in this game. Of course it was way more pixelated than this. But they even managed a cockpit panel :-). I wouldn't call it photo realistic but it didn't look half bad for the time. There are probably better/later game that this thing is shooting for. But I just never really got into combat flight simulation.
I guess they are trying to recreate some of that experience. Not that it matters; but I guess the gameplay is more important than the looks for this. Actually looks like it could be a lot of fun.
BTW. I have nothing against MS Flightsimulator. The latest version looks great. But I just don't have any windows computers anymore at this point. Combat is not really something either of the other flightsims I mentioned are made for or even good at.