You could get a GPD Micro PC. It does have a screen, keyboard and trackpad, but it's probably about as small as the device you were envisioning anyway (certainly smaller than a Steam Deck), and in any case I suspect it's a lot more practical to have some sort of built-in interface to such a thing, even if it's compromised. You could easily remove the screen and/or extract the motherboard, but you'd have a much less useful device for only a small win in portability. It must also be said that it's a deeply fun device.

A USB-C laptop shell that was nothing but a nice screen, a good keyboard+trackpad, and a generous battery would indeed be an interesting product, and a fantastic companion to either a GPD Micro PC or a powerful smartphone. (edit: someone posted about the "NexDock" products which appear to be exactly this. Hooray!)