The most important feature added in WordPress 5.8 is the Block-Based Widgets System. Basically, from now you can add blocks inside widgetized areas:
After months of hard work, the power of blocks has come to both the Block Widgets Editor and the Customizer. Now you can add blocks both in widget areas across your site and with live preview through the Customizer. This opens up new possibilities to create content: from no-code mini layouts to the vast library of core and third-party blocks. For our developers, you can find more details in the Widgets dev note.
How to disable this feature and revert to the classic widgets?
function themeprefix_setup_child_override() {
remove_theme_support( 'widgets-block-editor' );
}
add_action( 'after_setup_theme', 'themeprefix_setup_child_override' );
Add this code in your functions.php from your child theme. The result will consist in reverting back to the old/classic widgets dashboard:
Why switching back to the old widgets system editor?
Note: The 5.8 update is a major one and most probably 99% of the themes are not ready yet for it. This fix will stay active until compatibility will be added with the new widgets system.