Forums didn't have multiple HTML pages in the early 90s, they stuck everything into a database like they do now, and pulled it out with PHP3, or even Perl (the one I wrote was PHP-FI, then converted to PHP3, and is now entirely lost).
Especially the ones predating PHP. Also a couple of the most popular open source Perl ones. I remember Matt's wwwboard working like this, for example.
Some of them didn't. But A LOT certain did.