Hey there, I’ve been troubleshooting this exact issue with other developers lately—turns out YouTube rolled out intentional changes to their Iframe API that broke the setPlaybackQuality() function, and yes, even their official demo page is affected. Here’s what changed under the hood:

• Forced playback quality control is now deprecated
  YouTube completely removed the ability to manually force a specific playback resolution via setPlaybackQuality(). Calling this function now does nothing—no error is thrown, but the player will ignore your request entirely. This is a deliberate decision, not a bug.

• Adaptive quality is now the only way
  The player now exclusively uses YouTube’s internal adaptive bitrate algorithm to choose the best playback quality. This algorithm factors in your user’s network speed, device capabilities, the video’s available resolutions, and even current server load to minimize buffering and optimize viewing experience. Developers no longer have any control over this selection process.

• Related API behaviors have shifted
  Even getPlaybackQuality() has changed: it now only returns the quality the player is actually using (determined by the adaptive algorithm), not the value you tried to set with setPlaybackQuality(). Additionally, the onPlaybackQualityChange event will still fire when the player’s quality adjusts, but you can’t trigger those changes manually anymore.

Quick Workaround Note #

If your app relied on letting users select playback quality, you’ll have to rely on the player’s built-in quality menu (the three-dot menu in the player UI) instead of building a custom control via the API. There’s no official way to replicate the old functionality right now.

内容的提问来源于stack exchange，提问作者Volodymyr Humeniuk