It was recently requested of me to update my Minecraft mod Cooperative Advancements to version 1.15.2, which actually surprised me a bit--I didn't think anyone really used the mod. I hadn't touched that code since I originally wrote it, and I found that there was a bit of a painful re-learning process to get it to build. I had to wrangle lots of different error messages (I prefer using Visual Studio Code, which still is not fully supported by Forge to this day), but I eventually got the mod building properly in it's original state. After that point it was smooth sailing porting it to both 1.15.2 and 1.16.5.
I wrote the mod originally for personal usage in custom mod packs, where there were huge collections of advancements that are daunting alone but make for a fun cooperative experience once you can work together with friends to work at them. For those of you reading that use the mod, I hope you enjoy using it in more modern Minecraft modpacks! Check out the new versions at CurseForge here.