Matt Dewald, SCI Santa Monica’s Technical Production Senior Manager, posted on the Steam community forum explaining the reasons behind lack of fullscreen support.
We made the decision to go with Borderless Fullscreen based on extensive performance testing and research into to the support capabilities of Windows 10 alongside the fullscreen optimizations done by Microsoft in recent years.
Unlike with past versions of DirectX and Windows, the improvements implemented into the DirectX 11 version that God of War runs upon means that Borderless Fullscreen fully supports VSync, NVIDIA G-Sync, etc.
Additionally, we concluded that the upscaling we built into the game (TAA, NVIDIA DLSS, AMD FSR) provided higher-quality upscaling than what a monitor could give running at lower resolutions for performance reasons.
We do recognize that there are some ongoing issues with our implementation of Borderless Fullscreen that we are currently working on resolving. For instance, HDR needs to be turned on at the Windows-level to work properly rather than at the application level, and sometimes the mouse doesn’t stay constrained to the game window. We will continue to make improvements towards future-proofing our execution moving forward.
[Source – Steam Community Forums]