Increasing user reports on Reddit and Google's Forums suggest that the Google Assistant's sleep timer feature for smart speakers is broken. There are a couple of workarounds to get it to work.
Sleep timer RSS
Spotify is finally updating its iOS app with the Sleep Timer feature that lets users set a timer to stop playback. This native solution can now be used in place of the Clock app to stop playback.
Spotify is reportedly testing a few new features for its Android version such as a Sleep Timer, the ability to share songs with nearby friends, and integrating music controls into Google Maps.