Can I fit it in my pocket? Can I make a call? Take a photo?

The steam deck is great, but it’s not build for those things.

And there is no reason to cede ground on entire form factors. This behaviour will not stop at phones (or tablets, or smartwatches).

The Mecha Comet appears to be pocketable and comes with a camera. I haven't actually used a phone to make a call when out of the house for like 15 years, so the whole "mobile phone" part seems pointless to me. Actually pretty much the only reason I have a phone number is for MFA. Add a GPS module (ideally built in, but could be bluetooth), and it seems like it'd just be an upgrade over a phone.

Anyway, there are also other Linux devices others have pointed out in the phone form factor like the pine phone, and tablet/notebooks from e.g. GPD. So the form factor is there, but you might need to look at things that are advertised as small computers with touch screens (because that's what they are). So "handheld PC", "pocket computer", etc.