During the v2T migration, the ESXi hosts are added to a group created by NSX-T called:
NSGroup with TransportNode for CPU Mem ThresholdThis group ensures that the transport nodes have the correct CPU memory threshold settings in NSX-T.
As this host is part of the group, it prevents the VIBs from being removed until the host is removed from the group.
This issue is outlined in the NSX-T migration guide as a warning that you should remove it from the group before trying to remove NSX-T from a host that was migrated.
https://docs.vmware.com/en/VMware-NSX-T-Data-Center/3.2/migration/GUID-CEA71E88-F7DF-444B-ACD6-DF20DB1D5674.html