What does non-supported broswer mean? Well, firstly surfers using IE6 won't be able to use all of the features available through YouTube. Secondly, if someone is using an older browswer, YouTube will give them a nudge to upgrade their browswer to something like Chrome, Firefox, Opera, or even IE8.
Google has already started fading out support for Google Docs as well as other Google Apps for IE6. Because of all the security issues, instability and incompatibility, the internet and even some governments have united against this browser.
If it were up to me, I would ban Internet Explorer in every form.