There’s a good post somewhere here on the forum that lists out all the items that have been created and are in the game files, but are not available to unlock, such as the Striker V3.
Then there are items that are in the game files because they were an unlock in H1 or H2, but have no current way to be unlocked in H3, such as The Florida Fit Suit or the Undying ET items.
Then there are usually items that are not active in the game right now, but will be used in the future. I (obviously) don’t have an example to share 
When we started the “we need to replace the HWK 21 with something else” task, we couldn’t add something new unless we patched it in, so we needed to use something that was already ‘ready to go’ and could be changed server-side (i.e without a patch). There’s only so many of those items and they don’t all have a planned way to be unlocked. The Striker V3 was a good pick becuase it was another pistol, has a bit of legacy behind it, not a complete reskin and also should be appealing to H3-only players who never unlocked the so-called V1 from H1 GOTY.
Hope that gives a bit more context.