Ideally you don't need to backup host. You just install it from scratch. VM configurations are backed up together with the VMs, and all cluster-wide things are shared amongst the cluster, so you need to have some kind of auto-provision for them, like an Ansible playbook which configures network and joins a node into a cluster.
This is, obviously, not the case for the single host. Backup /etc and /etc/pve which is a FUSE-based file system bases on SQLite file stored somewhere in /var and which contains PVE-related things. Finer selection of things from /etc is possible, see this PVE forum thread.
I ended up installing Proxmox Backup client on the host itself and backing up the whole root file system and important mounts (since it doesn't span across file systems) into a Proxmox Backup Server: /, /etc/pve, /boot. It's possible to extract individual files from such a backup afterwards. Such a backup tends to be around 2 GB and, considering the deduplication PBS provides, subsequent backups will consume far less additional space, so this is not a concern. PBS team has "backup for Proxmox hosts" in their road map by the way, so this approach must be close to the view Proxmox developers themselves have on the problem.
Never used those backups because both cases I did a host reinstall I used an Ansible playbook and there was a cluster.
