It's not in the form of a difficulty slider, but how the game lets you ignore enemies and bosses to come back to them later when you're much stronger.
There are ways to trivialize pretty much every enemy in the game.
In previous From Soft games, if you got stuck on a boss, while you could run around and level up, usually that boss would still act as a roadblock.
In Eden Ring, if you're having trouble with a boss, there are 50+ other bosses you can go fight instead.
Or explore stronger areas you're not supposed to be in and grab powerful equipment or spells. Because there are few times when the game really forces you to fight, until late game.
And also unlike previous games, there are checkpoints you can chose to respawn from in case you die, so you don't always have to run all the way back to a boss if you got killed by one.
Elden Ring also gives you back all your healing potions to maximum capacity when you respawn. Which means spending less time farming healing potions, and more time practicing the enemy you're having trouble with.
It also lets you chose how many HP potions you want, and how many MP potions you want.
If you have a total of 12, you can chose to have lets say 10 HP flasks, and 2 MP flasks.
Or since I'm a mage, I like to have a bit more MP flasks. So I would usually go with a 5/7 ratio against a boss. And 3/9 if I'm exploring the world.
This game can be very difficult, but it can also be very easy, depending on how you want to play.