Not sure why Proxmox themselves don't have an RPM build of the Proxmox Backup Client for EL 8 and 9, but at least there's a community built one. EL9 client runs fine on Fedora 41.

github.com/TomGem/proxmox-back…

Releases · TomGem/proxmox-backup-client

Proxmox Backup Client for RHEL 8 and RHEL 9. Contribute to TomGem/proxmox-backup-client development by creating an account on GitHub.

^GitHub