A lot of aspects of Magic: The Gathering can prove to be controversial. Different mechanics, cards, even entire expansions are argued about for years. And that can include card rules, too. Or, more specifically, The Legend Rule.
For the uninitiated, The Legend Rule means that when a player has two or more legendary permanent cards with the same name out there, all but one needs to be put in the graveyard because, you know, things can get too powerful. As of Innistrad: Midnight Hunt, the rule specifically says "A state-based action that causes a player who controls two or more legendary permanents with the same name to put all but one into their owners’ graveyards." It hasn't really changed since Magic 2014.
The Legend Rule is kind of like the tuck rule in the NFL, as it comes down to those who want it to stay in and those who want it to go.
Reportedly, a surprising number of Magic higher-ups and some players want it gone, as they see it as somewhat detrimental to the game. After all, if you play the game right and get more legendaries to come out, why shouldn't they come out?
The most notable person against the Legend rule is "Mr. Magic," Mark Rosewater himself. He not only wants it gone, but wants to introduce mechanics to build on players having more, such as the proposed 'unique' ability. That way, the really overpowered cards would remain in check and be limited, while others would be allowed to be out there in two or more.
"The problem is if we make a card legendary for one group we are limiting how many copies players who don’t care about it get to play. I’ve watched players open cards, get excited about them and go “Oh, it’s legendary” because it means it’s hard to build a deck around," said Rosewater of the rule.
However, the Legend rule is unlikely to go anywhere soon. Most players seem to like it, as does the majority of Magic R&D. Many see the rule as necessary to allow players a chance at not being overwhelmed, as well as build early on rather than plan a few big cards later in the game. This is especially notable for Standard and Modern players.
A few cards do slip through the cracks though, and specifically point out in their text as to allowing multiples of them while playing. This is a rare thing to do even today, but card like Delina, Wild Mage can easily break the rule. And yes, they are legal in most types of play.
The Legend rule still lives on today, but like our rules, not everyone is happy about it.