In case of Ubuntu 21.04 it seems that DKMS magic takes care of rebuilding ZFS support whenever I install a new kernel package (yes, even one that I've built myself from mainline git---currently running 5.12-rc7). Yes, it can require manual work, if zfs-dkms does not have support for the kernel one is running. This failure should be quite visible: installation of the kernel package will end in an error.

But more importantly, missing ZFS support should not brick the filesystem. The new kernel just can't mount it, which can be overcome by rebooting to an older kernel. And then addressing whatever the issue is.