Regarding ARC and MMIX we might expect some action from Joern and H-P respectively, but nobody is probably going to do the work for the others
AFAIK ARC has no plans to do work on the old ARCtangent-A4 port that is currently in gcc trunk. The ARCompact code is not suitable to be integrated into the trunk, since it would need a maintainer - see PR39303. I am personally not available as a maintainer for that port myself because my remaining time with ARC is limited, and when/if I find a new employer, there would likely be conflicts of interest if I maintained the ARCompact port. MMIX sounds like it could be fun, but I doubt I'll be able to spare the time and energy to work on it.
