Fairly recently the chip was studied in detail and replicated. But like most systems of that era, I believe it was circumvented fairly quickly by pirates. The easiest way would be to piggyback a licensed cartridge - unlike the NES, it has to be one with the same region and bootloader as there are several lockout chip variants, but that basic technique still works.
I never saw a piggyback cartridge at the time though, or one that was obviously unlicensed. Of course, this could have more to do with Nintendo's legal pressure than with technological possibility.
The main pirating route at the time (a device (oh which there were several) that sat under the N64 and had either an Iomega Zip drive or a CD drive for holding ROMs), had a port for inserting a real cartridge to use its lockout chip.
Yeah, though piracy is not the same as publishing unlicensed original games (bootlegs) like Xibalba 64.
As far as Nintendo's concerned, it is. Their method of piracy prevention at this point was putting a lockout chip on every cartridge, and attaching their license fee to the lockout chip. Your homebrew cartridge needs one too.
I think a lot of the early flashcarts were piggybacks. I was never into that scene though.