Hello Paul
In legacy IOS with older switches, when you configured an interface with the no switchport command and assigned it an IP address, the switch would internally create a VLAN (typically in the extended range) and an SVI associated with that VLAN. This was necessary because the switch’s architecture still fundamentally relied on the concept of VLANs and SVIs to handle Layer 3 routing, even though the interface was acting as a routed port. The show vlan internal usage command would show which VLANs were internally allocated by the switch for these purposes. The reason this command exists is so that you can check it before assigning new VLAN IDs, just to make sure that they don’t conflict with these internal allocations.
With the Catalyst 9000 series and IOS-XE, the architecture has evolved to better integrate Layer 2 and Layer 3 functionality. The new architecture no longer requires the creation of an internal VLAN and SVI when an interface is configured as a routed port. Instead, the routed port is handled more like a traditional router interface, which does not rely on VLANs or SVIs for Layer 3 functionality.
So since Layer 3 functionality is implemented differently (i.e. without the need to allocate VLAN IDs to an SVI), the show vlan internal usage command is obsolete.
Yes, that is exactly the case.
I hope this has been helpful!
Laz