Yes, like for here.
For newbies the categories won't be collapsed, so no problem for them.
No, I don't use What's new. I just browse the forums of my interest. I'm not regulary here so What's new is too much information of categories I don't visit
Many (visual) design elements in their current software are really impressively good imo and the forumlist overview is an example of that. Just fantastic design. I don't think any other forumsoftware can beat that overview design at the moment, including Xenforo. (However... XF2's overal UI/UX is better. And it seems Woltlab took several cues from XF to begin with... but that's an assumption I make).
Just look what happens when you collapse a category. Conveniently it shows you the amount of new/unread threads when your view of the forumlist of that category is blocked (since you collapsed it). Clever and so user friendly.
Now... XF2's style suits itself quite a bit more than XF1's style to have collapsible categories. So who knows this still can be implemented if the need is there? The reasons for such a neat function are clear (remarkably enough it only got 5 likes in the OP).