CYOA books are at least nearly on topic, as solo RPGs are barely on-topic (example of solo RPG stuff I think we would want to be on-topic). Embarrassingly, Lone Wolf Game Books are explicitly on topic as solo RPGs, according to that tag, despite this meta seeming to imply otherwise, that the apparent contradiction has existed for the last several years.
I think that CYOA games have always been kind of a gray area on account of they don't usually identify as RPGs but sometimes they do, and we aren't really very sure at all what an RPG even is.
CYOA books, however, as well as solitaire play and everything else relevant to this gray area, seem to be quite unpopular; we don't really get any questions on them or anyone arguing that they should be on topic, just because it seems like no one coming here is much interested in them. The fact that the meta sat for several years without ever really coming up helps to show the degree of non-interest our extended community has in such questions.
If we ever had a community subset demonstrate active interest in these questions being on topic, I think it would be appropriate to review the subject.