"PC" historically refers to devices that are "IBM PC" compatible, although nowadays that mostly means machines with x86 chips... except that powerful ARM desktops, laptops, and servers are becoming a thing too so that's not accurate either. Plus there's that whole "Mac vs PC" ad which also makes the term more confusing.
But even going by the recent historical usage, I'd say the Steam Deck qualifies since it has an x86 chip, whereas the PS3 has a weird custom PowerPC cpu (which, ironically, was made by IBM).
All consoles are computers, in the sense that their chips are turing-complete
Nobody has really come up with a computer that can only run things you like and none of the things you don't.
They're just computers locked down by digital rights management, opaque operating systems, or other protection measures.
I guess that depends on your definition, but really I'd lump it into handheld computer, I've owned several, such as the GPD Win series
You can install desktop Linux software on it with no need to perform any types of "jailbreak" so while steam os is a proprietary skin for Linux, its not really locked down the way traditional brick consoles are.
Console doesn't have a hard definition, so anyone could come through and make a case for why it is.
Edit: you can see the people replying after me all have different definitions and standards for the word, it's arbitrary really
They come locked from the factory and very difficult to jailbreak. If that's not forcing them to run a walled-garden then nothing is and words have no meaning.
Getting an Xbox into developer mode, booting retro arch, really whatever you want then doing literally whatever you want with it has never been easier. The 360 was far more difficult and continues to be difficult to hack and mod in meaningful ways. The series consoles you can crack open in like 30 minutes with an article and a YouTube video.
Steamdeck is more console than x86 PC is a platform. I get what you mean, but PS4 and PS5 are too technically x86 PCs. Most modern games' tightly coupled target are actually APIs they are using.
It can be one click in a compiler to compile the game to ARM PC, but it's a different story when you port your game engine to console, where you have to implement the same features using different APIs. (E.g. Raytracing, storing game data, connecting to profile, implementing multiplayer etc.).
In the example of SteamDeck, the platform is Win32 or Linux ABI compatible OS.