Hi,
the text you cited from the UCS App Center is a standard text which is shown for any App which is installed but not available for the UCS version you want to upgrade too – regardless if the App Vendor is Univention or not. In case of Nextcloud the Vendor is Nextcloud, in case of for example the UCS@school Apps (which are also not yet available for UCS 5.2) it is Univention – the text is the same. There is no political issue, it is just limited resources.
Nextcloud is working on providing the Nextcloud Hub App for UCS 5.2, the current status can be found here: Keycloak compatibility needed for UCS 5.2 · Issue #204 · nextcloud/univention-app · GitHub
For Proxmox: UCS runs perfectly fine on a Proxmox Hypervisor. Not sure what kind of integration or partnership you would like to see?
Best
Ingo