https://wccftech.com/stalker-2-unreal-engine-5/

It’s been known for some time that STALKER 2 will be running on Unreal Engine, as opposed to the custom engines developer GSC Game World have used in the past, but we didn’t know what version of UE was being used. Most assumed a tweaked version of Unreal Engine 4 was the basis of the game, as the current version of STALKER 2 began development somewhere around 2018, but it seems there was an upgrade somewhere along the way!

Today Epic posted an article hyping up future Unreal Engine projects, and the STALKER Twitter account responded, confirming they’re using Unreal Engine 5.