Jenkins currently uses YUI version 2.9.0 released in 2011. There is no newer version of YUI 2.x. YUI3 is available but was also set to deprecated in 2014.

The YUI library is heavy weight and the size of all loaded javascript is over 400k

The YUI library is hardly used in core and activities are already ongoing to remove the last usages of it.

Plugins should move away from using YUI and replace calls to YUI with plain javascript. In most cases the effort is rather small.

The complete list of plugins that will be affected by the removal of YUI is found here: https://docs.google.com/spreadsheets/d/1UjvtFmNmEdjMN5DUoFxJfBryA8q-E5_HwOzVKbVG9b0/edit?usp=sharing