JQuery

jQuery is a  library designed to simplify cross-browser development. MediaWiki has included jQuery since version 1.16.

jQuery Migrate
The plugin jQuery Migrate helps developers to migrate from jQuery 1 to jQuery 3. The tracking task to remove these warnings and improve the underlying code is.

Note that the stack traces are available together with the console log in modern browsers, to help locating the origin of the deprecated function.

ResourceLoader
As of MediaWiki 1.17 all resources are (or should be) loaded through loader>Special:MyLanguage/ResourceLoader|ResourceLoader. The default modules are stored in. There are no static minified versions anymore as ResourceLoader takes care of this when combining and optimizing all queued files. All code can assume that jQuery has already been loaded.

However, if your site skin was created prior to 1.17, ResourceLoader will not be loaded and jQuery will not be available.