As another data point, I've successfully upgraded four buster installs from 0.8.6 -> 2.0.3 as packaged from the buster-backports repo. My setups are similar to Daniel's (ZFS as root filesystem, following the same install guide); one instance uses native ZFS encryption while the other three don't. All four also use ZFS for the /boot/ partition. One install is on physical hardware with legacy BIOS, and the other three are virtual machines.
Mathias
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