Proxmox comes with baked in support for live vm migration[0], high availability[1] and clustering support[2] if you run multiple host nodes. I'm assuming that's what OP is referring to

0: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#qm_mig... & https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_guest...

1: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_how_i...

2: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#chapte...

It doesn't on its own but it simplifies things hard to roll by hand that does, like observation nodes, consensus algorithms, Ceph storage, live migrations etc.

Some defense against the class(es) of availability problems that occur from OS + upwards. No defense against the class(es) that occur below that (metal).