@vkeven XCP-ng 8.1 release note says VSS and quiesced snapshots support is removed, because it never worked correctly and caused more harm than good. Note that Windows guest tools version 9 (the default for recent versions of Windows if you install Citrix drivers) already removed VSS support, even for older versions of CH / XCP-ng I am not sure if this VSS feature is bound to the PV drivers, or if it also needs hypervisor support. Though it is not recommended to stay on a old version of the guest agent.

@vkeven you can request a meeting with our sales teams, but technical discussion are better here if possible