Why would you?

The important part is your VMs and LXCs. Your Proxmox server, at most, requires configuration for specific needs (GPU passthrough for example). The only thing you need for that would be notes on configuration (unless you have this automated, in which case you don’t generally need that, just the notes in your scripts).

So a more important question, to me, is why do you want to back it up in the first place?