Some of the impacted themes are Astra, Storefront, Hestia, Neve, ColorMag, and a lot more. On the plugin side, SiteOrigins Widgets Bundle, Advanced Custom Fields, AMP for WP, WooCommerce Services, Ocean Extra, and Rank Math SEO were some of the big ones that were broken by the change.
If you want to avoid complaints from your users, you should update your tool right away. Joost de Valk posted a list of the affected extensions in WordPress 5.5, which you can consult for your users’ safety.
If you rolled back to an older WordPress version after seeing the errors, you should update to WordPress 5.5.1 because this is the release that is meant to help you fix all the issues.
By the way, if you have any general feedback regarding the latest major WordPress release, fill out the form in this post.