5.7 completed the jQuery upgrade begun in 5.5 by removing jQuery Migrate, which had been silently patching deprecated calls for years.
// what Migrate had been fixing, invisibly:
$.fn.andSelf() // → addBack()
$.parseJSON() // → JSON.parse()
$(el).load(fn) // → .on('load', fn)
$.trim() // → String.prototype.trim
// and the shim, for a site that cannot fix its plugins yet
add_action( 'wp_default_scripts', function ( $scripts ) {
$scripts->add( 'jquery-migrate', '/js/jquery-migrate-3.3.2.js' );
} );Migrate had been logging every deprecated call to the console for two releases, which is the warning nobody read because a console warning on an admin screen is background noise. The breakage lands in plugin JavaScript rather than in core or the theme, so the site owner experiences it as “the gallery stopped working” with no obvious cause. Re-adding Migrate is a deferral and should carry a list of which plugins need it.