There was a small update on the development of S.T.A.L.K.E.R. 2 made today via the game's official Twitter account. Instead of utilizing the X-Ray Engine, or at least a highly updated version of it, they will instead be going with Unreal Engine.

The X-Ray Engine was the 3D game engine created by GSC Game World. The engine was used to power the first three S.T.A.L.K.E.R. titles: Shadow of Chernobyl, Clear Sky, and Call of Pripyat.

This news comes about 8 months since we last heard anything about this mythical sequel. In fact, the last we heard about Stalker 2 is when the game was announced for the third time. Even then, the devs realized that the release was a long way away, already suggesting a 2021 release window.

Based on the announcement, seen below, it seems as though the team wasn't yet ready to make the reveal public. Epic seems to have forced their hand accidentally.
Stalker 2 Unreal Engine

Given that the release is still a ways away, the team isn't yet ready to talk about what platforms or digital stores Stalker 2 will appear on. They say simply that they "will talk about this later." It probably isn't too far-fetched to assume that the Unreal Engine powered Stalker 2 will be a timed exclusive on the Epic Games Store. That is obviously a big assumption, but the pieces seem to fit.

Epic gives a larger than normal cut to developers that both sell their games on the Epic Games Store and use Unreal Engine. However, some updated Steam policies may mean that window of exclusivity will be as short as a month. This is assuming there is any sort of timed exclusive agreement set up in the first place. This is also assuming the development team wants to sell their game through Steam in the first place.