What does this preference mean?
And is it still relevant now that legacy shops are basically not a thing anymore?
Some people still have legacy shops even if you can't access the page anymore. They implemented a feature where you can complete quests by pulling items from your legacy shops (if you still have any) as a way of trying to help get people to empty them out and delete them, if you have any items the questers ask you for in there instead of buying them from shops.
If you don't have any legacy shops then that preference doesn't really affect you.