Another point just to add to the conversation on a usernames potential state, “No user matching that login.” being returned by the user card could also mean several things.
- That user name may be valid, never used, and available
- It could be previously held by an account that changed name, and so is reserved and not available
- It could now be for a Twitch account that’s a redirect and not an actual user, and not available for use
- It could be a reserved name, and not available for use
None of that can be determined by the API either.