Can you live link a youtube video in Keynote as opposed to embedding an offline copy?
No, keynote is cross platform (web, iOS and macos) and designed to not live-link to YouTube. 

Use another app. One to try us LiveSlides. 

Consider that you will still need internet connection to play the video because it is still linking to the website where the video is, if you do not download it, means that you might be in a situation where the presentation will not work.