Why does that happen?
One thing that caused all that error is deprecated support for jQuery Migrate in WordPress 5.5. And seems like a lot of themes and plugins were depend on that library.
So if your dropdown menu, popup, slider, gallery, etc seems no longer working, that probably caused by this jQuery Migrate issue.
It’s Not WordPress Fault
Even though the error is present after updating to the latest WordPress 5.5, I do think it’s not WordPress 5.5 that should be blamed for that, Your outdated themes & plugins do 🐒
Using the latest code and best practice is always the best way to keep your site working well and also secure. Outdated code can become a security risk for your website.
Even if it seems like a big issue, the fix is actually very easy. WordPress team has released a plugin to temporarily fix that issue by bringing back the jQuery Migrate to your website.
You can install this plugin from your WordPress dashboard, just search the name or you can also download from the plugin page https://wordpress.org/plugins/enable-jquery-migrate-helper/
Installing the plugin above only temporarily fix your issue but I still really recommend to update your themes & plugins code to works with the latest and some future versions.
If you are using a premium theme, you can contact your theme developer and ask for an update from them. In case your themes or plugins are no longer maintained or you are using custom themes or plugins.
You can also hire a WordPress Developer to help you fix that issue and make your site up to date and secure.