That said, there are some people modifying the proprietary firmware to disable DRM. They have only done a small number of all of the printers have have such chips in them:
Many decades ago when I was a unix admin, you used to just dump a postscript file to the printer device, and it would print. From a quick google search, every printer I see has postscript drivers - from hp to epson to brother. PS is open, you don't need a driver from the vendor to make a PS file - you can make it with any software you want.
So if this is true (and maybe it's not), then the reason there are no open source printer drivers is because there are, and they've been around always, and the same one will work for all printers, because PS is a standard that's not printer-dependent.
Many, if not most printers do not block 3rd party ink with firmware - it's the driver in windows that blocks that stuff. Many, if not most, block it with a driver in the OS.
Drivers can in essence have hotlists for additional filtering or be given access to communication with an ink chip by the firmware, but AFAIK downgrading a driver doesn't work because what has really happened is it updated the firmware.
A driver upgrade can also never be sufficient for blocking copying on a MFP printer, a standardized net print protocol as you mentioned or using open source drivers either on Linux or under WSL.