After Gutenberg 8.9, WordPress has completely replaced the old
Appearance > Widgets screen. This new screen is now called Block Areas. As a long-time user of WordPress, seeing for the first time, I was a little lost in the new screen and it doesn’t feel comfortable. Adding blocks in here is a bit different from working with the post-editing screen. The legacy widgets can be collapsible whereas these blocks can’t but the complete sidebars themselves can be collapsed. Also, there are some legacy widgets from several third-party plugins that are not working. Though the WordPress core legacy widgets are working well.
So as the block-based widget screen is in its early stages, for the time being, if you want to disable it place the lines of code to your theme’s
//ENABLES THE CLASSIC WIDGETS SCREENS add_filter( 'gutenberg_use_widgets_block_editor', '__return_false' );// Disables the block editor from managing widgets in the Gutenberg plugin. add_filter( 'use_widgets_block_editor', '__return_false' ); // Disables the block editor from managing widgets.