I wonder, with all the programs that emulate old video game consoles and dead operating systems, are there efforts to make programs that can install directly to the printer hardware to undo this type of stuff?
Yeah, but if my printer is pretty much a computer that can have its OS/Applications updated, I'm surprised that hackers haven't found a way to install software to make a printer be a printer instead of a "live service device".
Firmware, not software. It's a layer in between software and hardware. You are probably familiar with a most commonly known firmware terms used in PCs, called BIOS or UEFI.