I'm finishing my master's degree in physics and I'm searching for a PhD position. A potential professor for PhD is almost 60 years old and he has not published any paper since 2014!
What do you think about this?
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:
I suggest talking to professor and asking these questions face-to-face.