Most players back in Borg's time were using text book flat SH BHs. Some of the prominent players that used DH BH stood out. Jimmy Connors is probably the big one although technically his BH was a DH Forehand
Back in the day Borg's BH was considered to be a DH BH. I guess it depends on the POV regarding what defines a DH BH. Some people think it depends on the racquet take-back. Others think it depends on what is happening at the point of contact with the ball.
Borg's takeback on the BH was definitely DH. And the stroke path to the ball was also DH. He took his left hand off the racquet at the Release Point a la a Hockey stroke as mentioned by
@Lorenn. I also consider Borg's BH a ONE AND A HALF ... although technically I think it actually is a DH BH.
I don't think it was considered ugly though. It was very effective and generated a lot of topspin. Regardless, I think most people were more concerned about Borg's looks, his athleticism, and his massive success in the sport. He was the quintessential Tennis Rock Star!
(As an aside, Borg's FH was an enigma. He created an epidemic of tennis players around the world who would try to roll the face of their racquet over the tennis ball thinking that's what generated the massive topspin that Borg could generate. But high speed slow motion video analysis of that stroke - by late great Tennis Coach Victor Brayden - revealed that the ball had left Borg's racquet face long before he rolled it over. Borg was generating the massive topspin the same way Rafa does .. perpendicular racquet face meeting the ball at impact.!)