It is street art.
But you see it in galleries and museums. One South Florida museum has one of his altered phone booths. But doesn't it lose something in a museum--taken off the street, taken out of context?
Of course they do. And this led to this Banksy Out of Context show-wide exhibition at Context Miami. It is still art of course but it isn't as provocative and if Banksy makes a piece of street art now? It becomes instantly valuable. It becomes something that gets taken down and transported to some collector or museum.
Is this good? Bad? Neutral? Doesn't matter because it makes even the densest person think about the nature of art.