It doesn't apply to just basic lands, yeah. But any special lands you can only have 4. It's been a rule for as long as I've played (since 2017), but I do know it wasn't a rule at the start of the game. I think they added it pretty early on though, as a response to people making decks out of just channel and fireball for instant wins.
And, sure, you could keep the ratio of card types the same, but while I don't play Yu-Gi-Oh, I have to imagine there are some cards better than other cards. So to make a deck that big, you'd have to include cards that just aren't as good. Playable, sure, but I can't imagine it finding its best cards consistently enough to be competitive.