When using _ as an area name, an virtual all area is accessed. The all area is read only, and contains posts form all other areas.
Does this populate the area list from /areas/ ?
As the doc states the _ area is in addition to those listed in /areas/, so it is not listed on /areas/. It contains all posts in all areas from /areas/
Why is this done server side instead of client side?
I'll probably going to abandon this.
This will be implemented differently in one of my next diffs.
Trying to subscribe to a post in all area