It's at the very least a dark pattern.

There's no reason it needs to actually block the printing - it could simply pop up with a warning saying that the toner is low and attempt to print anyway.

I'm not sure about Brother, but I do know that some other printer cartridges contain an electronic counter in them that will declare them as "out of ink" after printing a certain amount of pages (even if there's plenty of ink left in the cartridge).