I am looking some like the popup camera of the Oppo F11 Pro that can be connected via USB-C. Here is an example:
I think something exactly this small would be impossible as most of its connections, board and circuits are inside the smartphone. So, maybe even a little bigger would be fine as long it would not be just impossibly massive like normal webcams are for my use case.
All I need it for is to scan Pokemon TCG Live codes from booster packs I open while not at home. So for video quality as long as a QR Code is readable its fine.
I don’t know if it works on SteamOS, but I use my deck in Windows very, very frequently due to using it as a main computer and I had a giant stack of years worth of code cards from the SUMO/XY era I never touched. I just used something called Camo to used my phone as a webcam and got through the stack pretty quickly.
I imagine there might be something similar for SteamOS, especially since you can use a phone as a mouse and keyboard with KDE Connection.