Hi Johannes, I think you misunderstanding the issue. Proxmox DM now already support PVE 6 (except PVE 5 because has no Token feature, so I agree with you to ignore this PVE 5 version), you can see PDM has option to use manual API Token in this case, what is missing is: Migration feature. Another feature like Stop and Start already supported for PVE 6.
Citation needed at least on https://pve.proxmox.com/wiki/Proxmox_Datacenter_Manager_Roadmap or in the first post in this thread nothing is said about supported versions. I might have missed a staff statement on this but imho it's more likely that just because something is (barely) working doesn't mean it's supported. And the cli feature remote-migration (with the framework under it) was introducted in 7.4 according to the PVE release notes/roadmap thus I doubt that there are any plans to backport that to older versions. It would need some work with nothing much to gain (because again it's a good thing to discourage people from keeping EOL systems running in production)