And this discussion from November has an update on the progress: https://github.com/fish-shell/fish-shell/discussions/10123

Wow, very surprised how quickly they got this done. Looks like it took about a year, total, for the rewrite? Speaks well of their choice to use the Ship of Theseus strategy to port individual parts from C++ to rust, rather than giving into the temptation to do a "full" re-write.