The following link would always return a flash player until not so long ago. I know this because I recently checked in-browser rather than in CEF when Twitch removed their old API:
https://player.twitch.tv/?channel=bobross&flash
I also doubt Twitch used a codec other than MP4 at some point. VP9 is too resource intensive to encode. I wouldn’t have ever had to find a Flash solution for the Twitch part of my CEF app. Even YouTube uses MP4 for live streams when Google is the ambassador of VP9 (like, they really really want to replace MP4).
@BarryCarlyon It’s not in my, and probably nor in OP’s power to recompile CEF with the proprietary codecs because the applications are not our own. Even if it was, it would be illegal to distribute it unless you come to an agreement with MPEG-LA.