Perhaps coming with the OpenZFS merge, documentation around zfs volmode has disappeared. Most recent release with the info appears here: https://www.freebsd.org/cgi/man.cgi?query=zfs&apropos=0&sektion=0&manpath=FreeBSD+12.3-stable&arch=default&format=html This is still relevant to post OpenZFS merge FreeBSD and should be included in the documentation.
It's still there. But it's been moved into zfsprops(7).