I noticed that I was seeing an "Object expected" error in sass.worker.js when running on IE11. It appears there is an issue in recent versions of emscripten that is causing this:

emscripten-core/emscripten#6204

They cite passing the -s LEGACY_VM_SUPPORT=1 to emcc as a workaround. I've tested this and it does work. Apparently, there is also an open pull request to incorporate a fix into default emscripten output.

Just wondered if in the interim it might make sense to add the LEGACY_VM_SUPPORT flag to libsass/Makefile.patch in order to restore IE11 support.