There hasn't been any intentional change to the layout. The Discourse forum framework we use is autonomously managed by the Discourse, and the updates to the framework do sometimes result in changes to the forum layout. So it is possible that is the cause.
A more likely cause is the recent restructuring of the forum categories (see: Updated web site 👍). This restructuring has solely been a matter of renaming, removing, adding, and moving categories. It has not involved any changes to the layout settings. However, the layout of the categories list does automatically adjust according to the structure of the categories.
The category lists that are now inaccessible on devices with a small display look like this when the forum is viewed on a display with sufficient dimensions:
Note that it is formatted as a vertical list.
However, some other category lists have this format:
Note that it is formatted as a horizontal, line wrapped list.
Some of the category lists which are now in a vertical list format were previously in the horizontal format:
I don't know what criteria determines which format should be used. There isn't a setting for this.
In general, I greatly prefer the vertical format as I find the horizontal format difficult to read and messy looking. But of course an ugly layout that is mobile friendly would be better than a clean layout that is hostile to users of mobile devices.