This feature has been marked as deprecated since Moodle 3.8
Yahoo! User Interface Library (YUI)
This feature has been marked as deprecated since Moodle 2.9
As of Moodle 2.9, the user of YUI is generally discouraged.
Why are we moving away from YUI ?
The decision to begin the transition from YUI to JQuery was made here:
This is in response to Yahoos announcement that they will be ceasing all further development on YUI (although it is still maintained). http://yahooeng.tumblr.com/post/96098168666/important-announcement-regarding-yui