I was looking for how to deactivate the widget blocks that appeared with version 5.8 of Wordpress and I couldn't find it here. As I found the answer elsewhere, I share it here.
So, to disable widget blocks, just add this to your child theme's functions.php file.
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false' ); add_filter( 'use_widgets_block_editor', '__return_false' );
If you don't use (or don't want to use) a child theme, you can install this plugin : https://wordpress.org/plugins/restore-classic-widgets/
Hope it helps.