Sometimes they are, sometimes they’re not, sometimes we never find out.
There are a bunch of people who’ve disappeared over the years for reasons well-known or not. Best you can do is try bumping their log by tagging them, cross your fingers and hope for the best.
You’ve been here just over a year. Imagine us old-timers who’ve been on the site 12+ years and seen tons of people come and go. It’s one of the downsides of forum-friendships.