I did a fesh install of PVE 6.3 (UEFI, ZFS RAID1), then update/dist-upgrade, reboot, then I did zfs upgrade rpool :
root@x:~# zpool version
zfs-2.0.3-pve1
zfs-kmod-2.0.3-pve1
root@x:~# zpool upgrade
This system supports ZFS pool feature flags.
All pools are formatted using feature flags...