S.T.A.L.K.E.R. 2 has had a tumultuous development cycle, previously being announced and then later cancelled in 2011.
An official website popped up early this morning with little more than a title and a date of "2021". The post says nothing more about the upcoming sequel or game, but you can read it over right here if you want to. However, not all is good news for fans of the franchise, because along with the announcement and confirmation of the title, its launch window was announced and Stalker 2 is scheduled to debut in 2021, so there are 3 long years of waiting still. As for what the unusual acronym means, it was apparently revealed to be "Scavengers, Trespassers, Adventurers, Loners, Killers, Explorers, and Robbers" back in 2012 when the first attempt at making S.T.A.L.K.E.R. 2 was underway.
Rewinding the clock 11 years back 2007 and we have the original STALKER: Shadow of Chernobyl released, pitting gamers into a first-person shooter set in the Soviet-era nuclear power plant of Chernobyl.
As for the game engine, nothing has surfaced yet. The project is now underway at GSC Game World. The site shows a dark concrete background with "S.T.A.L.K.E.R. 2" and "126.96.36.199." written on it.
S.T.A.L.K.E.R. 2 website landing page.