Does mixed damage falls of late game

no its just not as strong because its mixed

What do you mean by "mixed". IMO there are no true hybrids.

Against tanks it can fall off since it is harder to itemize penetration for mixed damage.

Against squishy champions it doesn't since they can't itemize sufficient resistences without sacrificing damage.

The reason that people say it falls off is due to the way mixed damage interacts with % pen. Penetration is better if you have a lot of one kind of damage; buying last whisper and void staff is not really efficient, and not building them means you don't do much damage...

The best thing Riot could do if they wanted to promote true hybrids would be to make a mixed pen item. The balance on such a thing might be difficult, but I think it should be possible.

dpends on the enemies builts. If they have tanks which get both armor and mr, then hybrid dmage falls of, even vs a squishy with GA(armor+mr) it does fall off a bit, as you have no choices for defens penetration, while raw physical/magical can get Void staff/ Last whisper. however, most hybrid damge champs have so much raw damage that they still do considerable damage to most targets even later in the game(jax is known for a strong lategame despite doing hybrid damage)

to make it short, mixed damage falls of ategame, but champs that use mixed damage often have so high scaleings that they still stay relevant late.