Here is the situation. ZFS in 15-PRERELEASE has feature org.openzfs:raidz_expansion. By default a new ZFS pool is created with all supported features on.
However, the ZFS bootloader doesn't know about it. So if somebody does a new install or a new root filesystem booting from ZFS they are unable to boot.
Now, adding this is the easy part. What I need ZFS specialists to say whether org.openzfs:raidz_expansion is indeed transparent for file read purposes. I imagine it could get complicated when e.g. an expand is in progress and interrupted by a reboot.