Greetings.
I have just checked, and this is not the case. If an actor is incapable of wielding a shield, it will not show up in his list of available items for the offhand slot. This is also true if you have multiple armor types for shields that are all equipslot type 1 (shield). The actor only has access to shields for which he has the traits to equip:
This image is taken straight from the demo project, where the default shield is a 'large shield' and the small shield is a 'small shield'. Both items are shields, but they are different armor types, and Harold has been changed to only be able to use small shields. As you can see, the default shield doesn't appear in his list of offhand items.
Likewise if you remove his ability to equip small shields as well, no shield shows in this list at all, even when both the default shield that Therese wears, and the small shield from the npc shop are both in the inventory.
