I remember using some of them to varying success, most notably freeforums.org, Zetaboards, Icyboards, and Forumotion.
Last i checked, the former three defunct long time ago, and Forumotion faded into obscurity.
What happened to these? Was it because times have changed, or was it because better websites appeared?
Forumotion is still going - whether you call them Forumotion, Forumactif or any of the other brand names they call themselves. Choice of hosted forums, choices of features, they never went anywhere.
Icyboards closed due to the owner having time commitments and not wanting to sell.
Zetaboards I believe sold themselves to Tapatalk during the latter's buying spree - a bunch disappeared during this time, including the once mighty Invisionfree.
The reality is that for a lot of these, they weren't super profitable, but they could make money during the boom years. Latterly, not so much.
Most from Icyboards were converted to Jcink. While sporks didn’t want to sell out (even though Tapatalk tried), he did offer the possibility to export the database free of charge.
I used ProBoards only once in my life and it didn't last long due to losing interest in the type of board I ran. I would be interested in giving it another whirl if I could think of a new forum I could run on the software, I would like to learn more about it.
Don't forget about Jcink, the forum software I currently use. It seems dead as a doorknob when you frequent some forums and many of the larger forums have closed down over the past years, but I gain anywhere from 30 to 50 members a month, surely there's still some interest in the software. One person joined my board's Discord channel recently and said my board was the only active resource board he could find, so I guess that's why I gained so many new members.