If you're running a rolling release, you should already expect this kind of experience.

If you run a stable distribution, you stay on the older kernel series until you upgrade the entire OS, by which time the release notes would say "Removed support for ReiserFS"

And then there you can do the "tar -c -C /reiserfs . | tar -x -C /ext4" dance (or whatever other preferred copying mechanism), before the OS upgrade.

Such big changes rarely happen overnight. If someone really knows about and runs ReiserFS, they can be expected to read up on release notes for major upgrades of their distro. And it's far from an everyday event to even consider deprecating a filesystem. The fact that we are having this conversation rather proves that.