From the recent Game Informer and IGN Top 100 RPG lists, as bad or fine as they could be, have shown that RPG's are quite varied lol
I personally think Breath of the Wild is inbetween the old adventure action games and an RPG. The clothing with benefits, how it can be upgraded, the crafting, all the quests, how not only can you improve health, but stamina...I think it's stretching its limbs more into the RPG realm than it ever has before, where the games before were always more just a straightforward action game. The shift started with Skyward Sword, as I believe that had like crafting materials you could pick up too.
I don't feel saying it's an "RPG" is 100% right, but it's close. I think it comes down more to the Roleplaying aspect of the term. You can't really choose how you want to play Link. You can't be like a focused archer or a mage or a swordsman or anything like that, because the game doesn't offer options for making Link be anything other than the Link he is. To me, Breath of the Wild is just an Open World Action Adventure game, but they could very easily turn the next one into a proper RPG and I'd be cool with that.