We provide our own repository with slightly modified upstream code. That said, if there is no technical reason, that would hinder building Ceph for the current Debian (Buster) release used in Proxmox VE, then Proxmox runs with upstream Ceph releases.
Little history behind this, while Debian Stretch was current stable, Ceph decided to change to a newer C++ version (17). Sadly the needed toolchain was not available for Debian Stretch. This held back the Ceph upgrade till Debian Buster (now current stable).