First party usage, powering the functionality of the twitch.tv site itself and other Twitch apps.
Much like PubSub, the design is primarily a first party product, which developers are allowed to have access to as a secondary concern but not the primary function. Unlikely products like EventSub, or Helix, which are products developed for the 3rd party community as Twitch uses separate services internally.
All 3rd parties attempting to use whispers have to do so within the limitations of whatever anti-spam measures, restrictions, rate limits, and other limitations that Twitch don’t disclose for obvious reasons. Because of this whispers are not a reliable communications medium for what a lot of people are wrongly attempting to use it for.