Has there ever been a non-trivial, stable, and functional anarchy?
Sure there are lots of small groups that operate on personal relationships and don't have what could be called a government. I don't dislike such things. I just worry that they are not candidate models for a country big enough so that everybody does not know everybody personally. And lots of anarchic conditions arise through some kind of war or disaster or some such, but don't last very long, or are very grotesquely bad places to consider living.
Let non-trivial be defined as roughly the size of or bigger than Iceland in population. (About 350,000.) Or 100,000 for individual ancient cities if you have an example.
Let's be a little relaxed about the definition of functional. Maybe something like, if the country you currently live in was like that, you would not find it worth while to move to get away from it. This feeds into the stable thing. If a place is so horrible nearly everybody wants to leave then it can't be particularly stable.
Let stable be defined as at least 25 years with substantially the same arrangement. That is chosen to be about one generation.
So, no kings, no oligarchs, no junta, no elected law makers, no priestly leaders handing out punishment for transgressors. Only all voluntary, all the time. Anything like that?
An explanation of why I put in those requirements (though they might seem like very high standards):
There are currently a large number of countries, using other political systems, that satisfy the requirements I set. I certainly would not label any of them perfect, but then, they are jammed up with people. What are you going to do? There are plenty of countries that big. Many of them have been around substantially in the form they are now for at least 25 years. And there are many where people don't suddenly en mass abandon the place. So lots of other political systems exist that people can tolerate for at least long enough to raise children, and that involve communities of millions.
So, if anarchy was a model political system for those communities, it would have to deal with those limits. If, say, it only works for groups of a few 100, then it isn't a model for, say, the USA, China, or Europe. If it could only be kept together for, say, 5 years max, ditto. And if nearly everybody who tried it wound up hating it, again, ditto.