While trying to add Author/Maintainer searching to the Addon Manager, it was surprising to discover those keys were not included in the JSON file.
It seems as through that file is much more abridged than one might have assumed.
(It also seems odd that the t:Type {Gramplet, Tool, Report, etc.}, a:Audience and s:Status are lookup indices instead of rather than the explicit name. This seems fragile.)
Can additional keys be added to the JSON without breaking the Addon Manager?
I’m not sure why we use single characters rather than matching the field names in the addon registration. Maybe the original author wanted to keep the file size small?
You should be able to add extra fields ready for v6.2.
Yes, the original author was worried about file size. 
Why? Well I was worried about using github as a distribution method for addons. I didn’t see much of that kind of usage back then. I didn’t want to cause issues with the JSON being too big, and making the listing download take too much time.
These are generated by the system, and read by gramps. They aren’t written by humans, so aren’t any more fragile than a zip file. But they are not self-documenting, and not easily changed. But adding new keys and values should be no problem. At least I made them JSON!