See this Twitch blog post for details
"Twitch Prime, the streaming video site’s Amazon-style benefits program, will no longer include ad-free viewing as a complimentary perk starting on September 14th, the company announced today. Initially launched two years ago as a suite of benefits for Amazon Prime subscribers, Twitch Prime has grown into a more robust platform that offers free games, in-game loot for select titles, and a monthly channel subscription credit that can be awarded to a streamer of the member’s choice.Twitch Prime members will lose ad-free viewing next month -- The Verge
All those perks are remaining part of Twitch Prime, and the only aspect of the service that is changing going forward is the ad-free viewing. Twitch describes the decision as a way to help better support streamers. “Advertising is an important source of support for the creators who make Twitch possible. This change will strengthen and expand that advertising opportunity for creators so they can get more support from their viewers for doing what they love,” reads the company’s blog post."