This game prepares no one for raid/hard content

[Enla]

The game teaches you a good deal. Even single encounters in most raids, especially the 24 mans and Extremes, go out of their way to teach you the mechanics you're going to face in a gradual fashion before they ramp up the difficulty later in the fight. An example would be Mustadio in Orbonne. His first 'Maintenance' cast only tethers two adds outside the arena which allows you to see the mechanic that's about to happen but have a wide area to dodge into when the animation goes off. Second time Maintenance happens he adds a new mechanic where two blue adds spawn and cover a quarter of the arena each, you can infer once the cast goes off that being in the same pie slice as those blue adds will hurt after you've seen the mechanic and will stay away from them the next time. Then the next time he uses Maintenance he combines the two mechanics. Since the game already showed you how to do both parts, you should be able to infer where you need to go based on the visual queues.

Then more recently there's the Hades fight. A lot of the mechanics from the normal mode are present there, with the added fun of 'double' and the Cleaver mechanic from Eden Savage 2, and a good chunk of the fight is basically recycled mechanics from past Extremes. If you've done the normal mode you can work out at least 40% of the fight on your own even without having touched any other Extreme content. Heck you could have only done the normal eight man raid stories and recognized a good chunk of the mechanics you need to perform and learn how they work in that particular fight.

FF14 is constantly reusing mechanics, the stack marker for example as others have pointed out is a staple even in the MSQ. It's just a matter of paying attention and being willing to take a few knocks before they click in any given fight. If you aren't sure about any given mechanic you can also always ask your party for help or look up a guide before hand.