We noticed at our web agency that the update to WordPress 5.5 often causes problems. After extensive research, we discovered the cause.
 
With version 5.5, WordPress started disabling so-called jQuery Migrate by default. jQuery Migrate contains functions that have already been removed from jQuery itself.
 
There can be various reasons why functions are removed. Either there are now better alternative functions that should be used instead, or the functions contained critical security vulnerabilities.
 
In theory, jQuery Migrate is not needed, but unfortunately, things look different in practice. Outdated plugins and (especially) third-party themes tend to continue using old functions. Disabling jQuery Migrate therefore causes these components to stop working. And that is exactly what happens when updating to WordPress 5.5.
 
The quickest solution, at least temporarily, is to install the Enable jQuery Migrate Helper plugin.
This makes it possible to re-enable jQuery Migrate on the website.
 
As mentioned above, however, this is only a temporary solution until the theme or plugins have been updated to the latest version by their respective developers. It is important to remember that jQuery Migrate may also contain potentially security-critical code.
 
It becomes problematic when it is foreseeable that the developer will no longer provide updates in the long term. In this case, it is strongly recommended to look for alternatives to the theme and plugins.
We encountered this issue at our web agency while updating the WordPress CMS to version 5.5.
