... using grub to directly boot from ZFS - such setups are in general not safe to run zpool upgrade on!
$ sudo proxmox-boot-tool status
Re-executing '/usr/sbin/proxmox-boot-tool' in new private mount namespace..
System currently booted with legacy bios
8357-FBD5 is configured with: grub (versions: 6.5.11-7-pve, 6.5.13-5-pve, 6.8.4-2-pve)