Missing DLSS option in AC: Shadows can be solved by upgrading your graphic drivers.
Restart
Restart your computer from start menu and launch the game again.
Upgrade Graphic Drivers
A new driver update for Assassin's Creed Shadows is up and you need to check your graphic's company app or site for it.
Launch from NVIDIA App
Download NVIDIA app [https://www.nvidia.com/en-gb/software/nvidia-app/] and launch AC Shadows from there.
Nope, that does not work. It randomly appears and disappears, both the framegen dlss and the upscaler randomly