Partnered Account for Testing Purposes

You don’t have to “reconstruct the Twitch API”, you can literally copy/paste examples from the docs to use as mock data.

Twitch announce breaking changes to documented services well in advance, so there should be sufficient time to make any required adjustments to testing as needed. The vast majority of developers (ranging from new developers to larger organisations) have handled the rare occasions where there has been a breaking change to the API without issue.

Amazon may be one of the largest companies on Earth, but Twitch certainly isn’t, Creating what you’re suggesting isn’t some simple thing they can throw out there, it takes development time away from other features, and requires time and effort to maintain. Twitch may be working on such features already for all we know as this is nothing new what you’re suggesting, but just keep in mind this is far from essential or critical as the majority of developers have integrated with the Twitch API fine for years, and so would be prioritised as such when it comes to where they choose to focus their resources.