The index are not even listed on the GW
stores once a codex is released for an army. GW
clearly intends to run down stock of them and chances are they will go off sale at some point, but might stick around as PDFs or the like.
They aren't? Looking through the online store a few moments ago I saw the Imperium 1 & 2, & Xeno 1 & 2 Index - print, EPub, & Enhanced. I didn't see Index: Chaos, but then I also wasn't looking for it....