Quite simply: you are running a plugin or theme that has outdated code. Why is My Site Broken After Updating to WordPress 5.5? As WordPress prepares to make the transition to updating to the latest supported version of jQuery, removing jQuery Migrate was a required first step to make sure plugins and themes are up to date and no longer using deprecated code. Older versions of jQuery are no longer supported, so it is necessary for WordPress to update coding standards. Why Did WordPress 5.5 Remove jQuery Migrate?
Query Migrate was intended to help developers during the transitional period as they replace their code with more modern code (or remove it entirely). Basically, jQuery Migrate helped old code run properly on the current jQuery version and later. JQuery Migrate is a javascript library that has helped theme and plugin developers preserve the compatibility of deprecated jQuery code. By turning jQuery Migrate off, many outdated plugins and themes no longer work properly, which can result in broken websites. WordPress 5.5 turned off jQuery Migrate, a tool that WordPress has included for many years to help plugins and themes with outdated code still run properly. What Changed in WordPress 5.5 to Break Websites?
