I had used nRelate for years as it was one of the few that allowed use of the first post image for the thumbnail without forcing me to upload the image to my install (I don't use a feature image because I host all my images on photobucket) then in the past couple months nRelate stopped showing thumbnails and I was getting the run around on their support site. I threatened to switch a day or two before they announced their shutdown.

I now use Contextual Related Posts

https://wordpress.org/plugins/contextual-related-posts/