Yes, that's got to be the most relevant Wikipedia article. Since it starts with two examples of spying on the UN that appear to have had zero consequences, I guess I was wrong.

I don't think there will be consequences but that does not mean it isn't illegal. Technically this is the same kind of breach that is against article 22 of the Geneva Convention on Diplomatic Relations (a pretty long document). That everybody does it all the time and that it should be expected is one thing, that the host country does it makes it something a bit more serious imo.

After all if the most powerful country in the world uses the fact that UN has a residence inside its own borders (but legally speaking outside of them) to achieve home-court advantage then it might be time to relocate.

But that will likely not happen.

Does "illegal" really mean anything in the international system? Certainly governments cite "international law" to score propaganda points, but to show that it really means something, it's not enough to show cases where weak nations were punished for violating it—one would need an example of a strong nation being punished for violating it. Without that, it isn't the rule of law, only the rule of the strong. Has there been any such case? (Certainly there are many examples of strong nations violating so-called international law with impunity.)

I think we'll know the answer to that question when oil finally really does run out.

There is too much uncertainty past that point to make any predictions at all but I figure as soon as the ability to project power short of ICBMs goes out the window and the geopolitical map drawn up around one particular resource gets redrawn there will be a big push for something more stable in the longer term. And trade embargoes are a lot easier to maintain when it isn't as easy to move around. Until then it will be status quo with bigger countries ignoring the treaties they've signed and ratified whenever they feel like.