WordPress 5.8 will turn the classic widget screen into the block based widget screen. While this is a transitioning move, it might create compatibility issues for some sites.
The following snippet can be used to restore the classic widget screen:
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false' );
add_filter( 'wp_use_widgets_block_editor', '__return_false' );
In case it helps, here’s how to add custom codes in your site: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/
If you’d want to use it as a plugin, you can download the Classic Widgets plugin by WordPress team.
That’s it. I hope this helps!