Yeah, but it is a shame that the rest is closed source. This game could seriously benefit from community contributions and/or from a fork. There is a really strange issue that occurs when the map is far too big; the ground shakes uncontrollably. Given how simplistic the game looks, you'd expect maps to be able to get really big without much of an issue, yet we have bugs like these...
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.
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
[2] https://forum.ysfhq.com/viewtopic.php?t=6374