My 20yr-old was a toddler when I joined, so not quite 20 years.
Everything used to be in what's now known as 'MN Classic'. Threads were read by scrolling all the way through and there were no pages.
For a long time there wasn't a limit on how many posts you could have in a single thread. The threads weren't that busy so there wasn't any real need.
A thread could stay on Active Conversations for an entire day, even if no one posted on it as it wasn't all that busy on here. That meant that there weren't all that many topics as they weren't needed.
People could pay MNHQ £5 a year to be able to CAT (Contact Another Talker). People who 'knew' each other would use MSN instead. Some people would post their email address on the thread and separate it out to make it less 'visible' online.
The OP's posts being highlighted only appeared in the last few years. MNHQ's posts weren't highlighted at all, and it was still possible to pretend you were from MNHQ by changing your name to one that was similar to Justine's. :o
It wasn't possible to post images at all, and there were very few emojis - happy, sad, and a grin I think.
Links weren't automatic, and in the early days there was some convoluted method to it involving brackets and various symbols.
There were no options to watch threads, so if you wanted to follow one you had to post something on it, even if it was just a full stop.
I still have mine set to MN Classic so may well be missing many of the newest features. I don't have things like bookmark or the ability to see who started a thread without clicking on it.