Disable Block Editor

You can disable block editor without external plugin by 2 lines of code in themes functions.php or create a folder as mu-plugins in wp-content directory and place a file and mu.php and add following line of code to it.

When you place code in mu-plugins folder they don't need activation they are automatically executed.

Below is code used in the setup.

add_filter('use_block_editor_for_post', '__return_false', 10);

add_filter('use_block_editor_for_post_type', '__return_false', 100);

Below are the screenshots used in setup.