I think this has always existed as far as I know. I don't remember it being explained in the Lodestone but I haven't exactly looked hard for it either.

People that played FFXI would know that once you can no longer get XP from a monster that it would no longer drop crystals. I'm not saying this is something you should just know as someone that may have or have not played FFXI; it's simply an observation since the two games do have some similar mechanics.