ghcHEAD (and the to be released GHC 9.6) have a JS backend that will replace ghcjs. We should get it to work in order to be able to get rid of the burdensome ghcjs 8.10.7. Ideally we can do this via the already existing pkgsCross.ghcjs infrastructure, but emscripten is proving to be a little annoying, so it may not be entirely possible.

Upstream build instructions.