There is no way to get all subscriptions of a user using the official Twitch API.
Using the documented subscriptions endpoints you can:
- Get all subscribers of a channel
- Check if a user is subscribed to a channel (with authorization by either the user or the channel in question)
The only way to get all subscriptions of a user is the undocumented tickets endpoint:
https://api.twitch.tv/api/users/:login/tickets
I can’t remember the required scope from the top of my head, but I can check later today if you need it.
As always with undocumented endpoints: Use at your own risk. The ticket endpoint is used by the website itself as well as numerous third party services like Streamlabs and BTTV (as I said, it’s the only way to get that kind of information).