I don't see the J Extension on here. Does anyone know what's the state of work of that group?
(J Extension is about dynamic languages acceleration; stuff like code caches, and maybe providing GCs some help. I guess that's new territory so it's not as straightforward compared to say, the bitmanip extension)
J extension is, as you suspected, not anywhere near ready.
The reasoning is simple. It is indeed relatively new territory. The research needs to be done, and standards should be based on solid research. This will likely take a significant amount of time.
Experimentation can be done using custom extensions, but only what's mature and proven belongs in RISC-V official extensions.
(J Extension is about dynamic languages acceleration; stuff like code caches, and maybe providing GCs some help. I guess that's new territory so it's not as straightforward compared to say, the bitmanip extension)