An overview of the important changes is available at:
https://jquery.com/upgrade-guide/3.0/ This page also contains advice on how to migrate your code. In most cases
it involves fairly
simple changes, such as using a different method name, or adding quotes in
The vast majority of the added requirements and removed methods are
the jQuery Migrate plugin with a deprecation warning in the console. As
such, it's unlikely
your code will require any immediate changes.
In MediaWiki 1.30, it will be easy to find uses of deprecated methods with
warnings. However, you don't need to wait for MediaWiki 1.30 to start
methods removed in jQuery 3 have been deprecated for a while already, and
replacements are already available in jQuery 1.x, today.
== Timeline ==
* April 2017: jQuery 3 lands in MediaWiki core master with jQuery Migrate
* April to July 2017: Testing and fixing of issues detected during
* July 2017: jQuery 3 enabled in Wikimedia's Beta Cluster. 
* August 2017: jQuery 3 enabled on Wikimedia wikis in production.
jQuery 3 will also be released as part of MediaWiki 1.30.0 (expected in
November 2017). 
As part of MediaWiki 1.30, inclusion of jQuery Migrate will made
configurable so that sites
that have already migrated may disable the plugin for better run-time