PhD with a supervisor who has not published anymore since 2014
You are concerned that this professor does not publish because she/he ran out of ideas and motivation. Although this could be the case, there are other possibilities, which I want to point to you:


They are working on a very large and important textbook and don't have time to publish.
They have a lot of PhD students and allow them to publish as a single author (a bit unusual these days, but still happens)
They don't cope well with modern technology (computers, word processors, simulation tools, etc), and rely on their students to help them put their ideas into  final publication.


I suggest talking to professor and asking these questions face-to-face.