The Steam listings for Warhammer 40,000: Sanctus Reach and Warhammer 40,000: Armageddon announce that each of these Warhammer 40K strategy games are on the chopping block, and will be delisted tomorrow (thanks PC Gamer). At that point they will no longer be sold, but will remain playable for those who already own them. Lest this spark fears of shortages, rest assured there are still more that 130 Warhammer 40K games listed on Steam. But if this still inspires FOMO, there's still a day to pick up one or both (on sale) to be able to continue playing after they are delisted. Additionally, word is Armageddon* and Sanctus Reach* will remain on GOG.com into the future, leaving a mystery to why they are being removed from Steam. A blog post from Slitherine does not shed any light on this minor paradox:

We want to inform you that the Warhammer 40,000: Sanctus Reach and Warhammer 40,000: Armageddon franchises will soon be removed from Steam and our website. Starting May 19th, the games and their DLCs will no longer be available for purchase on these platforms. However, this is not the end of the battle! Sanctus Reach and Armageddon will remain available on GOG.com, where you can continue to purchase and enjoy the full franchise DRM-free. If you've already purchased the game on Steam, don't worry — it will remain in your library, and you'll still be able to download and play it as usual. Thank you for your continued support