Re
Diablo 1 , has just Direct Draw Support , and of course no Texture Filtering , i think this will not work with Reshade , and too when it works , it will look then more Pixelated
But for all Widescreen Aspects , look here.
pcgamingwiki.com/wiki/Diablo
Edit , i have found this !
pcgamingwiki.com/wiki/Diablo#/media/File:Diablo_GOG_DirectX.png
also Widescreen will Native Work , with the GOG Version
You can check with MSI Afterburner Overlay , which API the GOG Version used , when it shows D3Dx expect 12 , OGL, then will it work with Reshade !