We're one of the channels that are affected by this change. Fortunately, our strategy has always been to maintain an archive of our highlights on both Twitch and YouTube. Going forward it looks like YouTube will be the sole home of our highlights.
We're currently discussing what to do with old highlights, and what to do with future highlights.
Fellow streamers: If you haven't started transferring your highlights to YouTube, now's a good time to start.
https://help.twitch.tv/s/article/video-on-demand?language=en_US#storage