It might not be updated often but I find QuickJS to be quite stable. I've been building on it for years with no compatibility issues whatsoever. It's well written, and I don't think the fact someone hasn't made a commit recently makes it be of any less quality or usefulness. Fabrice works on many projects and is quite prolific in open source, he's probably busy.

It says clearly on the GitHub mirror that pull requests aren't accepted; there's a mailing list that you can submit patches to.

If something works, why does it need to be actively developed ad infinitum to be useful?

Because JS is an ever-evolving language. ChakraCore has been "abandoned" by MS (technically transitioned to community ownership) for less than a year and it's already behind on ES6 and ESNext compatibility.

I've used QuickJS but when I got stuck on an issue (stack overflow in win32 because of passing by-value) I got no help from the tiny community on the mailing list and had to abandon the attempt and go back to (overweight) V8.

If ChakraCore is behind on ES6 compatibility, that has nothing to do with its recent abandonment. ES6 was finalized over 6 years ago (not to mention the time that was available to work on supporting it concurrent with the development of the actual standard itself).