You can turn level locking off in the settings for Duty Finder and solo old content like you would in WoW.
With the level locking on you can only get to the character level/item level they locked it to. If you do a lv60 Heavenward raid wearing lv70 Stormblood gear then you'll be automatically at the item level cap, so you don't need different gear for different expansions, the latest gear is "twinked" for everything.
The exception is materia melding where you get a slight boost by having the right crafted item level gear with extra materia rammed into it (there's an increasing chance for this to fail and only works with crafted stuff). Extra materia on higher level gear is just capped as normal, only gear within the cap keeps the bonus.
People only do that for the hardest difficulty content (Ultimate?) that very few ever clear and you can't take the level locking off those encounters.
If's a pretty good system, you can solo stuff to farm items if you like, but also do it at the old difficulty level to enjoy the fights mostly as they used to be and get bonus exp for them via duty roulette.