jQuery 3.5 in 5.6, and the deprecations that finally bite

5.6 completed a jQuery upgrade that had been staged over three releases, and the removals in 3.5 break code that has worked since 2011.

// removed or changed:
$.trim()          →  String.prototype.trim()
$.isArray()       →  Array.isArray()
.load() .unload() .error()   →  .on('load'), .on('error')
$.parseJSON()     →  JSON.parse()
.andSelf()        →  .addBack()

// and the shim WordPress ships, which logs every use
// wp-includes/js/jquery/jquery-migrate.min.js

The migrate shim is the mechanism that makes this survivable: it restores the removed methods and logs a deprecation to the console for each one, so the browser console becomes the list of things to fix. Reading it on a real site with real plugins is the audit, and the findings are almost always in plugins rather than in the theme. The shim is removed in a later release, so the console messages are a deadline.