The developer team at Proxmox have been equivocating as to whether they should go ahead with support but the Proxmox community effort for Arm64 seems to be solid.
We have a Lab full of Ampere servers for developement, testing and POC projects. I would love to have Proxmox utilize these ressources to add an official release. We host private clouds based on Proxmox for SMB and Enterprise customers and would love to offer Ampere as an option. See Alpha3 Cloud fpr more info.