Encryption can be overcome as can the ownership issue so you can get at the actual files, UWP and it's way of sandboxing and protecting the application is a bit more of a problem though so hooking into a Universal Application is complicated.
I believe MSI Afterburner and RivaTuner Statistics Server go via Application Frame Host.exe but hooking ReShade into the very core OS services and files seems like it would not be a good idea.
API isn't a huge problem I believe, Gears Remaster, Gears 4 and Quantum Break requires DX12 and it's possible Halo Wars 2 also use it but other titles use DX11 (Unsure about the Forza games.) but the way these programs work compared to traditional Win32 software is problematic.
I could be overlooking something though but I don't think it would be a easy task.
Shame, Sea of Thieves coming out shortly (Open beta soon too.) looks really interesting for example but it's UWP/Win10 store only with nothing announced outside of that platform.
(Though several titles have ended up on Steam over time aside from some of the first party releases from Microsoft though Killzone recently had a Steam version so it's not definitive either although I can't see Gears, Horizion, Forza or Halo landing anytime soon if MS is trying to attract to their store with exclusive games and apps.)
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