https://wccftech.com/steam-deck-4k-rx-6900-xt/

The Steam Deck console is capable of 4K gaming with an external GPU connected to the console via the M.2 slot, a new video has confirmed.

ETA PRIMA shared on their YouTube channel a new video that shows how it is possible to connect an external GPU to the console via the M.2 slot. No NVIDIA GPU seems to work as of now, however, so a Radeon RX 6900 XT GPU has been connected instead.