Followed categories API
It's possible for a third-party app to query a user's followed channels, but not followed categories. This is a very useful tool for creating alternate views to a follow list
Use-case: Raid recommendation tool - it compares channel stats to your own and recommends those that are most similar, and also those you haven't recently raided. To do this, it needs to start with a list of everyone you follow who's live, and then does further research.
Rosuav shared this idea
It is available in v5 /follows/channels ( results with "game" property ), but helix response contains only followed users