Create an item that decrease the xp recieved

[Jeeqbit]

You don't overlevel the MSQ that much unless you joined a server with the "Road to 90" buff which doubles quest experience, or did all side quests or did roulettes. If you do just MSQ and no sidequests, you can find yourself short of exp for the last 2.0 MSQ quest.

It does confuse new players at the patches, because they do not understand that 2.0 to 2.5 was once endgame content. So they can't understand why their progress came to a halt and they get 100 exp per quest.

In Endwalker, they changed it to not expect some FATEs and Sidequests to be done, so that if you do just MSQ you more or less have all the exp (they slightly undercalculated it, so do need to get a bit of extra experience). Due to this, you can level an alt job alongside it in Endwalker. But as far as I know, older expansions still need some extra exp than MSQ like they originally did.

An item to reduce exp gain wouldn't make sense. I've seen it suggested before by new players, but I just tell them that being overleveled does not matter, because their real level is the level of the MSQ quest. They cannot do any content that is higher than the level of their MSQ, nor access areas and duties higher than the level of the MSQ, except maybe Unreal. And they sync down to near the level of their MSQ.