If it never physically existed, how could someone have it in their collection?
Unless they can show they have the prototype, I think a cancelled item should be removed from anyone's collection. There could be an argument for having it in the stash as a reference...someone searches for "did anyone ever make a collectible of xyz" would show it, but as a product that never actually launched.
Conversely, just remove it entirely from the database. If someone can prove they got their hands on the prototype, that could then get set up as a custom one-off collectible.