In general my upgrade to WordPress 5.5 has been pretty seamless, across all of the client sites that I maintain. This is good news!
One of the main issues that I’ve come across is with a few of the sites running older themes (despite my recommendations to let me update or create a custom theme for them!). This can cause issues because in version 5.5 they have removed jQuery Migrate from the codebase.
jQuery Migrate was created by the jQuery Foundation when they released a new version of jQuery, which removed a lot of functionality – the “migrate.js” file added a lot of this functionality back in with deprecation warnings, in order to aid migration.
Unfortunately, some themes and plugins are still relying on the really old jQuery code that has long since been removed. This is a bad sign that the theme or plugin is not being properly maintained, and should therefore be avoided.
As a short term fix, the WordPress team have released the Enable jQuery Migrate Helper plugin, to add the “migrate.js” file back in. However, I would still recommend moving away from any theme or plugin that requires this, and to remove the plugin again as soon as possible.