The only useful information I have is in the following GeekList, where a BGG Admin is quoted as saying:
So, it sounds like in the past, any user proposing new categories/mechanics was asked to list 100 games that fit the criteria. If at least 100 games fit the bill, the addition is considered.
If you don't feel like going through that work, you can also feel free to post up a Geeklist and farm it out to the user base!
Let's add Social Deduction as a category on BGG!