Method 1: Do you want to disable the new widget block editor page of Gutenberg and bring back the old widgets page? You can do that simply by adding this line into your theme's functions.php file:

// Disables the block editor from managing widgets in the Gutenberg plugin.
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false', 100 );

// Disables the block editor from managing widgets. renamed from wp_use_widgets_block_editor
add_filter( 'use_widgets_block_editor', '__return_false' );

Don't forget to save the functions.php file.

Method 2: If you don't need to edit the functions.php file of your theme, install and activate any one plugin, and the old widgets page will be back:

1. https://wordpress.org/plugins/disable-gutenberg/
2. https://wordpress.org/plugins/classic-widgets/

Tested & working for me.

Hope this is helpful for you.

Thanks in advance

To disable the new WordPress widget editor system you can use one of following methods.

1. Install and Activate the Disable Widget Block Editor plugin.

2. Use use_widgets_block_editor filter to disable it. You can place following code in your theme functions.php file or your plugin.

add_filter( 'use_widgets_block_editor', '__return_false' );

3. Use following code in functions.php of your theme to declare that your theme doesn't support the new widget editor system.

remove_theme_support( 'widgets-block-editor' )

// Disables the block editor from managing widgets in the Gutenberg plugin.
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false', 100 );
// Disables the block editor from managing widgets.
add_filter( 'use_widgets_block_editor', '__return_false' );

It appears that one of the filters has been renamed. It is no longer "wp_use_widgets_block_editor", it's just "use_widgets_block_editor". The most upvoted answer by @Savan Dholu should be edited to reflect that (I'm afraid I can't comment as I'm missing enough reputation * ROLLEYES *).