It doesn't make a lot of sense to me to see a game with subtitles set to n/a for when all of the dialogue is text-based.
Say I'm hearing impaired or I want to play a game at work but remain aware of the real world around me, or have any other reason to not want to hear any dialogue - I may use the wiki to look for games that have subtitles. If we had a list for such games, I would naturally sort to highlight the ones set to true, not something as obscure as n/a. You may convince me otherwise, but the real question is whether it would be as apparent to a newcomer.
Besides, by dictionary definition subtitles do not have to accompany any voiced dialogue. One of the two Merriam-Webster definitions for subtitle is