One of the questions we get asked fairly often in the Queue and other places is "Is Blizzard working on an 'appear offline' feature for the Battletag/Real ID friend's list?" I understand the desire for the feature: Sometimes I don't want to have everyone I know see that I'm online because I just want to chill and do something small-scale or by myself.
Sadly, it seems that feature is currently having some troubles being realized. According to Lore, it's hit some technical difficulties.
I'm fairly sad about this, and I hope they can eventually solve those difficulties and get this into existence ASAP.