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.