If you search UserVoice I’m sure there’s a feature request for a dev account or sandbox that you can upvote https://twitch.uservoice.com/forums/310213-developers
Honestly though, the docs are sufficient for the vast majority of cases, even for things behind restricted features such as channel points, bits, or subscriptions, for many of us 3rd party developers to successfully create/test/launch apps on Twitch for years. If you have an app that is polling for channel points redemptions, or an EventSub subscription, it doesn’t really matter if it’s mock data generated by Twitch for a dev account, or mock data generated by yourself, as in the end it’d just be a simulated redemption passed to your handler functions.