That being the case, an unused revenue stamp seems to be a rarity despite being popular with collectors or not.
Yes, and *actual* unused 1st-3rd issue revenues can be worth a premium to the right collector. However, the stamps must have a large part original gum and not have any document offset, residual adherence, or other evidence of having been attached to a document to be considered unused.
A stamp without a cancel but no gum is "used but not canceled" in my opinion. This criteria seems to differ from front of the book material where "mint no gum" or "unused no gum" is apparently a thing.
With 1st-3rd issue revenues, without gum is not the same as unused/mint. It may be worth a premium to the right collector for aesthetic reasons, but it is NOT unused. Clerks were in a hurry, and in many cases, affixing the stamp was deemed "close enough for government work". Once that stamp is soaked/steamed from the document it appears unused.
Full-gum mint 1st-3rd issue revenues are fairly scarce. Regumming of 1st-3rd issue revenues doesn't appear to be prevalent, as there isn't sufficient premium or demand to justify it.