Ask YouTube to copy the old embed code, and it should still work (at least in 2013).
On the video's web page, you can click the Share link, select Embed, then choose the tickbox [x] Use old embed code
. The code it generates can be pasted into PowerPoint's Insert Online Video section
Ex:
<object width="420" height="315"><param name="movie" value="http://www.youtube.com/v/oHg5SJYRHA0?hl=en_US&version=3"></param> <param name="allowFullScreen" value="true"></param> <param name="allowscriptaccess" value="always"></param> <embed src="http://www.youtube.com/v/oHg5SJYRHA0?hl=en_US&version=3" type="application/x-shockwave-flash" width="420" height="315" allowscriptaccess="always" allowfullscreen="true"></embed></object>