htmx is a very small and very predictable library. The underlying pattern is well established among many libraries and probably countless similar implementations.
I have implemented something like htmx several times before htmx came out, and I assume many others have too. I just didn't take the time to extract a clean, generalized and comprehensive library.
I personally found a glaring performance issue in the library related to plugins and created a github issue for it back in December (I don't think it's been fixed to date). I was confused because it was pretty obvious. "Battle tested" things, by definition, have been through battles and don't have as many easy pickings on optimizations.