I don't know, I'd figured that by 2080 something like the Ottawa Treaty would be enforced worldwide,
It will hold exactly as long as the movers and shakers are those who expect to invade other, rather than be invaded, and not a day longer, quite obviously. Realpolitik.
but if someone can come with a reasonable explanation of why UN sponsored PHALANX would be allowed to deploy anti-personnel mines
Some beasts are more equal..?
— within urban areas no less! — something like that could be implemented, assuming of course any technical issues can be solved...
Once it can be made to specifically affect aliens, why not.
There are lots of non-urban areas.
Also, this raises the question of remote-controlled devices - as previously discussed, even unarmed remote camera would be much more useful than a blind mine. Reasonable middle ground between mines and HWPs could be worth deploying.