The two-column view on /categories
is pretty new, AFAICT. I am not sure what led to the change in that design but it’s probably documented in the Discourse discussions. We can certainly change the default view to /categories
vs /latest
if people generally feel better about it. 
For what it’s worth, the “hamburger” menu in the upper right does allow someone to navigate between categories and the top-level category view. Design-wise, it could still use some help IMHO.
With each category, e.g., at https://forums.librehealth.io/c/projects/lh-ehr you can see the latest posts from that category, and can also use those grey buttons in the upper left to switch out between categories.
That said, Discourse is also open source and we’re running against their master branch. They are generally very receptive to feedback and willing to make changes if enough users agree.