Here is the thesis paper that came out of it. https://dl.acm.org/doi/epdf/10.1145/3276479

All the following is my opinion, and may contain errors. Take it with a grain of salt and kindly correct me where I misstated something.

If you view PyPy as a sophisticated test bed for virtual machine research it was a success. There are lots of conclusions about what works and what doesn’t, and benchmarks that show faster code when using many core. But the project ended with the conclusion the the STM model and the tracing JIT model don’t play well together, and the PyPy team preferred to stay with the tracing JIT.

Python and Ruby are very similar. The same JIT that powers PyPy powers topaz, which still performs quite well in ruby benchmarks even though it has been dormant since 2017

My recollection (which could be completely wrong) was that they had a branch with it, but it really needed hardware transactional memory to memory to have decent speed for most applications.

Makes sense - they seem to have stopped work on STM so presumably it couldn’t be made to work well.

How will STM in Ruby solve (or avoid) the problems that doomed STM in Python?

It exists, it's just quite slow. I doubt there are users of it.