setPlaybackQuality() does not work anymore, even on official YT Api Demo: https://developers.google.com/youtube/youtube_player_demo
What has changed in the work of the YouTube Iframe api?
So, finally, answer from Google:
setPlaybackQuality is now considered a "no-op"; calling this function will not change the player behavior. The player will use a variety of signals to determine the optimal playback quality.
Users are able to manually request a specific playback quality via the quality selector in the player controls.