Do the tests require processes for any reason? Otherwise I don't remember explicitly turning them off.

@hoodmane can confirm, but IIRC, on Emscripten, the restriction is the lack of threads (or, at least, the decision to disable the limited thread support that is available on Emscripten because of security and compatibility concerns).

Yeah, threads don't work very well with dynamic linking on Emscripten. I think as long as you first load dynamic libraries, then start threads it's probably okay. But it's complex to support and I don't personally care enough. Subinterpreters don't seem like they can be all that useful without threading.

There are definitely some use cases for subinterpreters outside concurrency. There's some discussion surrounding this on the PEP 734 acceptance yesterday. I think the clearest one is that you can change sys.path to allow conflicting versions of dependencies to be loaded in the same process.