Fabled will not dissapear, if you already have it. The Fabled NPCs will stop popping, this is true, but if you have already looted your stuff, dont worry about it.. so go get what you can. Make it quick.
The Fabled jboots drop from the Fabled Drelzna in Najena, just like the old jboots dropped from the regular Drelzna way back when. They aren't quested. This ring exists for it's own sake, nothing else. If you don't like/want/need the stats, let someone else loot it.
I was hunting AC for a while now, and he eventually spawned as normal, didnt hit any higher than usual, and he dropped regular ring. I heard a rumor that fabled stuff will dissapear soon anyway so i geuss im fine