I do not know. I have been around for 19 years. During that time, there were several board software changes. "The big switch" in 2008? was the end of the world, just like the Invision switch February 2017. Anyone remember that right column? There was a switch to turn it off. I expect everyone who did not read that post on how to shut it off, had to live with it for many years.
People have been coming and going since the forum existed. Read some of the really old threads. There is a ton of people who left many years ago. Most all didn't give a reason why. They just stopped posting.
Prior to 2017, there was no reaction feature. When one was added with Invision, the number of posts declined. That's because people didn't have to quote someone to let them know how they felt about their post.