There has been a preview of some new Sepuchural Guard, but for Shadespyre which is dues late this year.
It could be a mistake on the website. While "Sold Out" usually means something has gone for good, it could be an admin error, and that it should be marked as "Temporarily Out of Stock". If it's still the same way after a week or two, then it could well be sold out.
Even then it could just be sold out in it's current form, as Baron suggested, quite a few of these have been shortly before re-packaging for Age of Sigmar on round bases. I would possibly expect that to go along with a Battletome though, and there hasn't been much of a hint that anything has been coming for Deathrattle, though I wouldn't rule it out entirely.
Over the next week, we'll start to see some new leaks for the next month, as we get closer to the preorder date for the new White Dwarf for May