When and why to deprecate filesystems

That doesn't help much. In the distant future you won't be able to compile then-ancient kernels (i.e. those with reiserfs still in them) with then-current compilers, just like you can't compile now-ancient kernels with gcc 12.

It doesn't matter (much) whether that kernel shall compile to a mere FUSE helper that exports a reiserfs file system, or a full-blown kernel you'd run in a VM (mount the reiserfs image there and export it to the host via NFS or CIFS). Assuming it'll work in that then-current VM; that's mostly likely, IMHO, but by no means assured.

The solution is (assuming that there is no reiserfs support library you can use a shortcut) to extract a libreiserfs from the kernel and remove all the kernel idiosyncrasies from it. At that point it's standard C plus libfuse and thus presumably easier to keep up-to-date with whatever fun incompatibilities the C and/or FUSE people will throw in your path in the future.