Shut down by whom? For what reason? What's usually illegal in these cases is distributing games and BIOS roms. Making and distributing things like emulators seems to be OK legally. I'm pretty sure this was tested in court via Sony's lawsuits against Bleem, but I understand there are always finer points that may not be apparent.

Also interesting and only tangentially related is that MAME used to have a rule about not supporting games within 2 years of release.

What? Why does that matter? Even Nintendo knows it has to leave ryuujinx alone. And switch is even a current platform.