What are the criteria that determine if a research group/professor is actively pursuing research? While searching for research groups pertaining to my area, I came across one that matched my interests well. But in the publications page, the most recent paper was written in the year of 2015, which was also the last paper by the professor. There are no indications that the group has been discontinued. As for the professor, he seems fairly active until last year (but no papers), according to his bio. I tried to get in contact with the university regarding the activities of the group but I didn't get any response yet. 

As for the students in the group, there isn't much information about their recent work. The university is pretty well ranked and my research interests align with the group so I'm hesitant to let this university slide but since papers are good indicators of how active a group is, does it mean it's better to move on? I haven't contacted the professor personally regarding this since I wasn't sure how to go about it. (I came across this and am considering sending him a mail.)

In general, are publications the ultimate measures of performance or is there a useful rule of thumb for choosing or not choosing of a research group/professor for PhD (apart from the publication list)? I'm not very knowledgeable about the process yet so any help on this is appreciated.
You have a piece of evidence that doesn't tell you very much. But there is a simple way to find out what you need to know. Send a message to the group's PI and express your interest in that area and ask whether they are still doing research in it. Is there any, more recent, work that they can share with you? Are there students in the group with whom you might correspond. 

It could be anything from "we have ten papers in the pipeline" to "we dropped that a few years ago". Or maybe even "we think we are on the brink of a major breakthrough and have been quiet about our work while we refine it." 

But only they know the real situation. Maybe they will be able to honestly share enough to let you know whether to move on or pursue it. 

I'll note, also, that the lack of publicly visible productivity doesn't necessarily equate with a lack of productivity altogether. Research breakthroughs can't be scheduled. If they could be, then it wouldn't be research. 

Note that according to Wikipedia, Andrew Wiles worked almost entirely in secret for six years before coming up with a proof of Fermat's last Theorem. There were a few glitches initially, but he was Knighted for his breakthrough.