So? The OP already stated that it was an unconditioned space, and doesn't use it. Clearly that means that there little to no hvac air flow, and he's fine with that. Closets are usually not considered habitable either.
Where's that excerpt from anyways, you didn't cite the source. Since we all live in different jurisdictions, this matters, as it probably does not apply to many of us.
Lol the concept of this code snippet applies to everyone as we all tend to need to breath.
Anyway its from the international code. sorry i didnt cite it. International code is adopted in all fifty states in the u.s. OP profile says new york. new york could possibly be stricter.(would prob if anything require more ventilation.)
The closet may not be habitable, but i would bet that the vent was to get air to flow out of the basement, which is probably habitable, into the closet and out whatever vent or return air is on the other side, or was there in the past.
the house is old as sh!t so, there is a possibility none of us will ever know what the vent was for or if it was connected to some other vent in the past. the vent cover it's self is old as sh!t you can tell by the style. they dont make em like that anymore.
So the purpose of me posting the code was to try and provide some context to why the vent may or may not be needed there and also to help determine if it can be removed.
Again my apologies for not citing properly.
IRC Link to code post in previous post.