Robocraft 2 Community Servers
The sequel to the successful Robocraft game featured more advanced building and even a (hidden) world editor. After the game received negative reviews after its early access launch the company decided to rebuild it in a different direction. The studio (Freejam) then eventually closed.
Using my experience with Freejam games and help from other players I've enabled the dev-only world editor for everyone and created a mod to run a local server (normally only official servers existed). When the game shut down I extended my mod and web server to get the game working again by implementing basic account management. I eventually implemented support for the CRF2 as well as a completely custom map API that allows creators release new versions of maps without affecting any ongoing match. I also created a mod that turns the game client into a dedicated server with lots of configuration options.
Modding became much more challenging after the developers turned on IL2CPP because unlike regular compiled C# code (IL) the result is machine code (with some metadata). So there is much less information available on what the code actually does and modding tools have more bugs and limitations. But these challenges made modding even more interesting, and it also made certain things easier (referencing private members).