Hi there,
The plugin should work fine with WordPress 5.6, but if you see any problem please let us know.
The “Tested up to:” version number that the plugin shows at: https://wordpress.org/plugins/regenerate-thumbnails/ indicates that the code has not changed since the plugin latest update. This generally means it was unnecessary to change it to make it work with newer versions of WordPress.com , and/or there has been no bug fixes/enhancements.
Best,
Thank you for confirming Gabriel, much appreciated!