If you are watching a repository, you receive notifications for all threads by default. Use this endpoint to ignore future notifications for threads until you comment on the thread or get an @mention.

You can also use this endpoint to subscribe to threads that you are currently not receiving notifications for or to subscribed to threads that you have previously ignored.

Unsubscribing from a conversation in a repository that you are not watching is functionally equivalent to the Delete a thread subscription endpoint.
Set a thread subscription