I think Gamefreak has put varying levels of effort into competitive balance. They didn't really try at all for the first 4 gens, they had some vague ideas with gen 5 (introducing HAs to give many previously bad mons a purpose, nerfing sleep) but they made a lot of mistakes (allowing auto-weather abilities to spread without nerfing them). Gen 6 is the first great gen competitively, when they had a picture of how they wanted the game to play and how to balance things, the first generation in which every Pokemon they added to the game had some obvious intention with its functionality, and our formats have been pretty clearly defined since gen 6.

They balance for singles and doubles both, but their idea of singles is 3v3. You have to be real about older formats. A lot of smogon's bans are historical going back to gen 1, but consider the balancing work that even made those bans a reasonable direction to go. Even if we grant in gen 1 you were probably supposed to play with Mewtwo and Mew banned, what was the intended strategy to take down Minimize Chansey? A faster sleep user? Gen 2, did anyone even consider how offense was supposed to work, and did they do any math to conclude anything could actually kill a Snorlax? Gen 3 they didn't even contemplate that Wobbuffet might hold Leftovers and fight another Wobbuffet. Gamefreak really wasn't trying back then so those historical formats are more things that happened because of pre-gen 6 jank. This is before we get into just how horrible the old gen balance really was (just look at the design of Dodrio vs Fearow vs Pidgeot vs Farfetch'd and how straightforwardly Dodrio outclasses all of the others... they don't do stuff like that anymore). The game is way more complex now with a lot more actual design work going into the balance; it's not really realistic to expect something as arbitrary as the number of Pokemon you use to remain static when you move past that sort of ancient Smogon era.