BulletML was developed by ABA Games (aka kenta cho, creator of the classic rrootage, which is now open source).
It was rrootage that sent me down a rabbit hole and eventually stumbled onto fal-works and firedancer after someone suggested their works had the same aesthetic appeal.
The libbulletml c++ library is still somewhat used in some niche vertical shmup games (mostly Japanese indie games). You can read a little about bulletml and the history of it all in the libbulletml readme file.
[0] https://en.wikipedia.org/wiki/ABA_Games
[1] https://github.com/abagames/rrootage