The Flash player (player.md) is being replaced by the HTML5 player (embed-video.md) thus the documentation was removed and it’s use is no longer encouraged. If you really need the old documentation, check back in the commit history.
As for prior notice, the Twitch blog announced the start of the process to move away from Flash back in July 2015: https://blog.twitch.tv/video-player-controls-now-in-html-109db74f38f6#.2j16dhr88