I have no DLSS option in the pull down menu in Diablo 4 only 2 AMD options and an Intel option, I’ve tried scan and repair which found nothing wrong and I’ve deleted the Localprefs file resetting everything, the nvidia files are present in the directory of D4. I’ve tried several different drivers to no avail and am back using latest ones.
I use a MSI RTX 4080 SUPER VENTUS 3X OC running on a MAG 341CQP QD-OLED via HDMI 2.1 @175hz.
Update:
So I rolled computer back 3 months and brought it back up to date, took a good while 
in order to see if that sorted the problem, anyway fired up BN after 4 hours of patching machine and there was an update for D4 so had to install that then tested the game and DLSS option is back, now I don’t know if the patch fixed it or the roll back of the comp did lol but its fixed.