The Yu-Gi-Oh! card game has been around for years at this point, and it has changed pretty drastically throughout its life. With the additions of synchro, Xyz, and Link monsters, cards have fallen in and out of favor.
There is a huge amount of cards that used to be fantastic but have weakened over the years. Some are still good, but all of these cards used to be incredibly powerful in their heyday. Nowadays, they just don't see as much play as they once did all those years ago.
Jinzo
A staple in Joey's deck from the anime, Jinzo was a no-brainer for pretty much any duelist back in the day. It had a high attack stat for only one tribute and disabled trap cards; what else is needed on a monster?
Jinzo could easily cripple any opponent's strategy with its effect, but nowadays, as trap cards have become less utilized, Jinzo has fallen out of favor. If traps one day return, this monster likely will, too.
Summoned Skull
Yugi had it right with this one. Despite being a vanilla monster, Summoned Skull was a shoo-in for any deck because it had the highest attack points in the game for a one-tribute monster.
While this wouldn't ever fly in the current-day game, it was just right for the early meta of the Yu-Gi-Oh! card game. Poor Summoned Skull receives no play nowadays, though.
Monster Reborn
Monster Reborn isn't terrible or anything nowadays; it just isn't the fixture that it once was in the card game. There's a reason that everyone in the anime had this in their deck. It was one of the only methods to special summon in the game.
Now there are plenty of archetype-specific cards that do what Monster Reborn did a little bit better. This card isn't banned as much as it isn't the absolute powerhouse that it once was.
Harpie's Feather Duster
Harpie's Feather Duster is simply a back row wipe to the opponent, plain and simple. It's a great card even nowadays, but it was actually only taken off of the ban list somewhat recently due to the power that the card used to have.
It's still one of the best cards on this list, but it had previously reached a former glory that few cards are able to do. Mai knew what she was doing with this card in her deck.
Polymerization
Polymerization isn't on the level that it once was anymore simply because there are so many retrains of it. Virtually every relevant archetype in the game has a version of Polymerization that is better than the original card.
Other versions also work in the same way but better. Essentially, Polymerization has been taken over by brand-new versions of itself.
Change Of Heart
One of Bakura's favorite cards in the Yu-Gi-Oh! anime is Change of Heart, a card that allows the player to control one of their opponent's monsters for a turn.
The real problem with this card is that if it were unbanned, there would simply be too many cards that can take an opponent's monster in the game. With Brain Control being unlimited, it would just be a bit much. It doesn't have as much power as it once did. It is still a very strong card, however.
La Jinn The Mystical Genie Of The Lamp
La Jinn the Mystical Genie of the Lamp used to be played constantly because it was the best level-four powerhouse in the game. While cards like Gemini Elf would eventually come out and surpass it, in the beginning, La Jinn, the Mystical Genie of the Lamp, was one of the better cards in the game that could be brought out so easily.
Now, this card sees absolutely no play because it's worthless.
Magic Cylinder
Magic Cylinder used to be good in a metagame that doesn't at all resemble what it is today. In a meta where trap cards literally can't exist, this card is bad.
However, once upon a time, Magic Cylinder could really do some damage. At this point, the only thing it would have going for itself would be the surprise factor.
Sangan
Sangan, just like some of the other cards on this list, is still a very good card; it just isn't at the level that it was.
With the ability to retrieve any monster with less than 1500 attack points when it's destroyed, Sangan was a beast, and it is still good. It just isn't the absolute monster of a card that it once was.
Dark Elf
Dark Elf is one of those cards that's level four with 2000 attack points but has a cost of 1,000 life points to attack.
While Dark Elf has nothing going on now, neither utility nor strength, she used to be a good option to quickly get a strong monster out onto the field. She simply isn't relevant anymore nowadays, unfortunately.