Justification: user security hole
This was assigned CVE-2013-6780:
Given that upstream states
| YUI 2 is an end-of-lifed project and is no longer supported. All YUI 2 .swf
| have been removed from the Yahoo CDN. If your site was taking advantage of
| presence of these files on the Yahoo CDN they will no longer be available.
yui should be removed from unstable. Please file bugs against it's rdeps.