What do you mean by "16-bit," exactly? The raspberry Pi's CPU is 32-bit, and so any piece of code you write to run on it will be by definition 32-bit. If you mean something in the style of old 16-bit consoles, that's a visual style choice, but has nothing to do with the actual "bitness" of the code.
As far as releasing on itch, there are plenty of engines and tools that are available here, and I don't see any reason why you couldn't release a Raspberry Pi OS as such a thing.