Keep in mind that legality only matters within a jurisdiction and that everything is illegal somewhere and at some scale, privately doing this is almost never illegal but doing it at industry scale would be. Next, try asking your lawyer if the factual law has much sway over the commonly shared understanding of the law (in a lot of countries it does not (e.g. you cant insult the monarch in most constitutional monarchies according to the exact word of the law, however this is not enforced nor cared about). Lastly, its only really legal till Nintendo throws enough money at it being illegal (there is no money in it for them, unless its about brand protection).

To conclude: the real answer lies in that the presets given from the Atari vs Nintendo case in America state that clean room reverse engineering of these chips is legal.

PS: IANAL, also gameboy games would be a trademark violation because of the security involving the nintendo logo being rendered on the screen by the cartridge and then being compared to one from ROM, a personal fav trick from nintendo also physically applied on the disks of the famicom disk system which held the nintendo logo on them physically indented allowing a stamp inside the disk reader to be inserted into it (easily bypassed by great groups such as hacker limited by simply adding a flat piece of plastic to the 2.8 inch quickdisks, or sipmly adding instructions on how to add this yourself).

A lot of this gets bypassed using ye olde method of selling the means and then making the user do the bypass themselves using the means.

I think the CIC chip would be clean-room [1] designed in this case. It's been done with the Nintendo 64 chip as well via decamping-microscopes. [2]

[1] https://en.wikipedia.org/wiki/Clean_room_design [2] https://www.youtube.com/watch?v=HwEdqAb2l50

If the patent is expired, why wouldn’t it be legal?