... read the whole story at github.com.
You can't satisfy all libraries, frameworks in all world and galaxy far away, etc., because in such way you will loose readability of language itself. It is not how it's suppose to be.
Libraries must change their methods. Not opposite. Language on a first place. Authors of websites must stop using old libraries. This is evolution. Old must die. Period. Website made on 8 year old technologies will stop working? Ok. Fine. Goodbye. I don't believe that Google or Microsoft will not implement this feature, only because of users of MooTools. We even don't know what percent of websites uses MooTools. And, surprise, anyway you can use old browser for this site.