Sorting the items of the API response on the client side is “not a big problem” only if a user doesn’t have followed hundreds or thousands of channels and the response remains below the request limit of 100. Otherwise you’ll have to chain API requests with increasing offsets, cache the results, filter out possible duplicates which may have occured between the requests, sort the entire list and then return it in chunks (or as a whole). This is nonsense and can’t be in the interest of everyone.
I too would appreciate if we could get an official response from one of the Twitch API devs. Having a streams response sorted by the date the user has followed the channels is just bad and doesn’t make any sense. Also please remember that the API endpoint is /kraken/streams/followed, which already implies that it should be sorted by viewers, just like /kraken/streams. If this change was intentional for whatever reason, then please consider adding a sorting parameter, so that the previous API behavior can be restored. Even though Twitch has deprecated the kraken API namespace, breaking changes like this should not be introduced.
Thank you.