I've been using the iframe embed with control=2 parameter as described by the docs for years. https://developers.google.com/youtube/player_parameters
Today 7/20/17 all the placeholder thumbnails have stopped showing.
I can still see the thumbnail in the resulting css code as a background image url. The Thumbnail though just doesn't show.
So to recap while the following still show a thumbnail
<iframe width="560" height="315" src="https://www.youtube.com/embed/1sO5OKez0JQ?rel=0&controls=1" frameborder="0" allowfullscreen></iframe>
Changing the value of control to 2, does not show the thumbnail anymore
<iframe width="560" height="315" src="https://www.youtube.com/embed/1sO5OKez0JQ?rel=0&controls=2" frameborder="0" allowfullscreen></iframe>