What it is in FFXI: Parties can use "level sync", which fakes everyone's level down to the sync target's level. Player stats, equipment, and abilities are capped at that level.
Example: I'm level 45, and my friend is level 20. We want to level together for a while. So we form a party, and turn level sync on, with him as the target. Now I'm a pseudo-level-20, as well, and I can no longer use the abilities that were learned at level 21 or later.
I think the reason they didn't go with it here is that.... they can't, really. The armory system lets a level 10 Thaumaturge equip a level 50 Conjurer ability, if that character has Conjurer at level 50. As a level 10 soloing, I wouldn't necessarily be using level-10-max abilities across the board, so there's no real way to enforce it, that I can see.
There probably are ways to do something similar, though. I just can't envision them as someone who has no experience in game development. I would think SE could come up with some ideas, though.