I started thinking about it last night after you posted again. The format of the old myBB forum pre-dates myBB and stuck with the world a really long time. I can trace the "Category / Board / Thread" format way back into the 80's, we didn't even have internet connections yet! Instead it was dial-up BBS (or Bulletin Board Systems).
BBS systems at the time were mostly single user affairs; that is, only one user at a time could "call-in" and connect. A very tedious method that left most of us playing with paperclip dispensers while our computer and modem auto-dialed forever to busy signals.
The next rendition, and first internet version of the "Category / Board / Thread" could be found in usenet newsgroups.
Finally all that morphed into the modern internet "forum". Still it all worked largely through the same familiar "Category / Board / Thread" format.
Features like "view unread" and "view today's posts" got added to popular platforms to make it easy for users that like to "follow the conversation" to be more efficient.
What's making all that change now? more computing power. Specifically the ability to "search". myBB was really bad about search, it still "needed" the "Category / Board / Thread" format so that users could self-classify information.
Today, with modern search, we've lost the need the for that kind of classification system. We can just tap the magnifying glass icon and start typing. Search is so speedy that the need for "Category / Board / Thread" formats has evaporated. Now we can simply lump everything together, perhaps throw in a few "categories" and/or "tags".
In this case it really is! As I said above, after less than 1 week I really like this implementation and never want to go back.