this is in from Proxmox

Today at 08:50: no, only the no-subscription repository is affected. a package was moved a bit too early yesterday and it seems the retraction was incomplete, we'll see about getting this fixed ASAP.

Today at 09:15: please just hold off upgrading for the time being - the check aborts the upgrade early enough so that there is no urgent action that needs to be taken while we are fixing this.

24 minutes ago: pve-no-subscription repository should be fixed again.

if you followed the advice here and have installed proxmox-backup-client 3.2.5-1 to work around the warning, please be aware that that version has opt-in experimental features that might not yet be ready for prime time. unless you explicitly invoke vzdump or proxmox-backup-client with parameters to enable those features, it should still be safe.