There are posed portraits and there candid portraits. The latter description is not an oxymoron, it’s a label I have for portraits that are captured in a photojournalistic style. They’ve been around since Cartier-Bresson invented the photojournalism and, of course, the former has been with us since the invention of the camera.
Since a photo portrait is, by definition “a likeness of a person, especially one showing the face, that is created by a photographer” – does it matter that it was captured on the fly, without the express participation of the photographed? Yes & no.