Would it be possible to have a category field added to the site object. Eg Technology, Culture, etc. The sites seem to already have categories, as you can see on this page: https://stackexchange.com/newsletters. (although it seems few sites aren't on that page?)
Categories aren't sufficiently stable to expose in the API.
It is entirely possible (even likely, in my opinion) that we'll revisit and rework the proposal categorization significantly in the coming year.