It's still unlimited (with a minimum) for constructed as far as I'm aware, except EDH at exactly 100. You just have to be able to shuffle the entire deck unassisted without resorting to piles and stuff. It would also be a terrible strategy to dilute your best cards since there can only be four copies. I'm sure there's some crazy gimmick deck that works with 250 cards that only Shaq can play.
If you take 4 60 card decks that all have synergistic opening lines..... That's 240. Add 10 one drop utility cards and boop you have a 250 card deck. I've done it in arena and they are very fun to play.
I started off when the game was still brand spanking new; they had no limits on either deck size or what could be in it. Half the cards I own are now banned in tournaments because I haven't bought new cards since high school (I graduated in 2003 for reference) and I had thought the deck size was limited, too, now but another poster points out that is still unlimited for the most part.
According to a quick google, you can only have up to three of the same cards in a Yu-Gi-Oh deck. So you can't keep the ratios the same.
I don't play Yu-Gi-Oh, but I play Magic, and it's similar there, but you can have up to four of any card.
I imagine most trading card games are like this, otherwise you could just make a deck of only the most OP card or something. Not exactly fun to build, or play, or play against.
The deck was a miller, basically every card was centered around forcing the other player to draw. Most cards that make your opponent draw also have you draw, but with a 2222 card deck you just draw and draw and draw, until they had no more cards to draw, then they lose.