Since [1] packages through -rc4 and [2] already has rc5 and 0.8 final in the upstream branches, I would suspect you'll be able to build a package for buster Soon(tm), though I do not personally know how creation of -backports works for testing during a freeze.

[1] - https://packages.debian.org/source/experimental/zfs-linux

[2] - https://salsa.debian.org/zfsonlinux-team/zfs