Helldritch
Hero
On the contrary, it has everything to do with being relevant.Whether it was optional or not is irrelevant. Whether you believe it to be popular or common or not is also irrelevant.
Come at a table playing second edition. Do they allow that book? Do they play in Planescape? If not, is your cleric allowed? If a single no is answered to these, you will not play a philosophical cleric as these do not exists outside this book or that specific setting. A DM will be within his rights to tell you no, that cleric is not allowed.
More over, during the few tournaments of 2nd edition, as per TSR, only the core books were used as "official" material for tournaments... Even owners of the IP were not allowing splat book either. The last "official" tournament I have seen was in 1993, 3 years after that book was printed. Never have I seen it used in any tournaments that I have been in or heard about.
The problem with claiming that such and such is canon with splat book is that not everyone owned it. In fact, some people found that book horrible, others like me found that book to be the best written of them (class related that is). And others simply did not have it. I do not remember the sale's figures, but it was not that big of a deal back then. Without a good income, many would simply pass on a lot of books, concentrating on their personal favourite line of books. Be it Ravenloft, Planescapes, Darksun, Forgotten Realm or Al Qadim or whatever, it was a rare thing to see someone owning all books that would come out. The only way to have a common ground was to stick to core books in discussions.
Now if you want to speak about a specific book. Feel free to do it. But do not claim it to be core. It was not. Far from that.