What are backports?

Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution
Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!

source https://backports.debian.org/Instructions/

Enable Backports

Add to /etc/apt/sources.list

deb http://deb.debian.org/debian buster-backports main contrib non-free

Install zfs

apt update
apt install linux-headers-`uname -r`
apt install -t buster-backports dkms spl-dkms
apt install -t buster-backports zfs-dkms zfsutils-linux zfs-initramfs

Tips

If you have problems when changing kernel

# new kernel header
apt install linux-headers-`uname -r`

# ex. use zfs/0.8.4 for /usr/src/zfs-0.8.4
dkms -k `uname -r` install zfs/zfs.version

# rebuild the initramfs
update-initramfs -k `uname -r`