I don't think, i try this morning but the option to Add Proxmox Backup Server on PVE was missing, so i upgraded my PVE.
You don't need susbscription to upgrade it, use the "no subscription repository".
# PVE pve-no-subscription repository provided by proxmox.com,
# NOT recommended for production use
deb http://download.proxmox.com/debian/pve buster pve-no-subscription
And dont forget to comment the line in /etc/apt/sources.list.d/pve-enterprise.list
The devs stated that they will not backport to 5.X versions as they are EOL anyway. You should upgrade to a more recent version Most of the time this can be done without a reboot anyway.