Why was jQuery Migrate removed? Andrew Ozz stated that jQuery Migrate is a helper tool that should be included externally from themes or plugins and should not part of the libraries bundled in the core of WordPress. Furthermore, this removal in 5.5 release was planned for the first step to introduce the latest jQuery script to replace the current version (1.12.4) in WordPress 5.6. This was a long-waited proposal since it had been raised from June 2016 back.
If you can confirm this jQuery Migrate removal breaking your site’s layout, try to install and activate the Enable jQuery Migrate Helper plugin to quickly fix it. If still, the issue remains after installing this plugin, you may contact your theme and plugin developers to ask whether they are aware of this and assure their theme/plugin is fully compatible with WordPress 5.5 or not. But before doing so, try flushing the cache on your site (if enabled) as it may enables web browser to load the cached version of your site. You may also need to clear cache, history, and cookie on your web browser end.