They might drop packages between now and the stable release. An official Debian release won't generally drop packages unless they've become totally unusable to begin with.
We manage anything, including package builds, ourselves if the need should arise; we also monitor Debian and release critical bugs closely, we see no realistic potential for any Proxmox relevant package to disappear, at least nothing higer compared to that happening after the 9th.
FWIW, we got staff members that are also directly involved with Debian which makes things a bit easier.
Given that Proxmox operates their own repos for their custom packages and users don't typically install their own packages on top of Proxmox, if a package they need gets dropped due to RC bugs (etc) they can upload it to their own repo
