If you go to the start of the head of the list of topics listed under this heading, there is a new post by Admin. it explains the changes and that you have to subscribe to the categories you want to read.
Alternatively, if you to to the Main list of topics, scroll down and click on All sections, all the old categories are shown. It means that lots of new categories can be added and we can subscribe to the ones we want and not see the ones we do.
Hope that helps