As you can see in the upgrade intructions:
https://pve.proxmox.com/wiki/Upgrade_from_8_to_9
In the section
"Systemd-boot meta-package changes the bootloader configuration automatically and should be uninstalled
As Proxmox Systems usually use systemd-boot for booting only in some configurations (ZFS on root and UEFI booted without secure boot), which are managed by proxmox-boot-tool, the meta-package systemd-boot should be removed. The package was automatically shipped for systems installed from the PVE 8.1 to PVE 8.4 ISOs, as it contained bootctl in bookworm."
If you update to Proxmox 9, remove this package