It's the other way round. As Google seeks to keep Youtube up-to-date it's no longer bothering with
older[ versions of Internet Explorer but Youtube should still work on more recent (post-XP) versions of IE.
However many web companies are now recognising that Internet Explorer is much less popular than either Firefox or Chrome, so compatibilty with IE is often a very low priority anyway.
The vast majority of the 'regulars' here in AB's Technology section (myself included) wouldn't touch Internet Explorer with an infinite number of the proverbial bargepoles. Firefox is the preferred browser of most ABers:
http://www.mozilla.org/en-GB/firefox/new/