That was for items that were over 400 stones in weight. They changed that like a year or so ago, I had a bunch of heavy stacks of regs and leather just sitting on my floor for a long time and they were poof after some patch. Didn't bother telling us they were changing it either. *grumbles*
Some items are naturally set to 'no decay'. Like vet statuettes and some 8th anny items like the tap of sosaria, etc. There are a few EM event items like that as well, for events where players would have to comb areas of the shard and pick items up from the ground. Some blackrock may be like that. Very old items that existed in the world before such a thing as lockdowns can be like that as well, tho if they are newbied they are usually fast decaying instead. Rubble is like that too I believe, from what I've been hearing about it.