Disregarding the parent comment, but hosting a soft fork is easy enough but it'll quickly become a spaghetti mess of local patches that conflict with upstream changes. It's not like there's an argument for preserving access to Russia either since the nature of the kernel being hosted across torrent trackers makes it impossible to deny Linux to any one country.
It seems like the better solution (imo) is to work on a different kernel receptive of these maintainers, so that the companies employing them can still have a kernel that is developed for their use-cases whilst supporting projects that don't so openly collaborate with hostile states.
Why wouldn't they be able to. Russia has a lot of tech talent, and tends to top programming competitions. Also, if this happened I imagine other countries like China would collaborate as well. China alone has a bigger population than all of the west, and a better education system to boot.
No, but they can host the infrastructure so that excluded developers (the ones that just so happen to be Russian) along with whomever will want (BRICS developers for instance) can surely contribute.