https://github.com/smarr/RoarVM is the main thing that comes up when trying to learn about this, and it hasn't been touched in over 10 years. I have also seen people working on spawning child VMs to handle parallelism, though unfortunately I don't have links to those discussions immediately at hand. It seems like perhaps an unnecessarily heavyweight approach, too, not a canonical solution that you would want to use in production.

I assume that step 1 towards parallelism, at least on the image side, would be going through the class library and making sure everything is thread-safe. I'd love to know where one would even get started with that effort. The Roar project claims to support Pharo 1.2, which doesn't seem to be very far after they forked from Squeak, but obviously a lot has changed since then. And the challenge is that Pharo is still rapidly developing all the overhauled classes that distinguish it from Smalltalk-80.

Meanwhile, if I want to play with parallel image/REPL-based programming, I can go over to Common Lisp and, while lacking an equally coherent GUI, be able to load up bordeaux-threads and off I go.

No, but you can do concurrent programming in Pharo: https://books.pharo.org/booklet-ConcurrentProgramming/pdf/Co...