Disable Gutenberg (Block Editor)

<?php
// Disable Gutenberg on the back end.
add_filter( 'use_block_editor_for_post', '__return_false' );

// Disable Gutenberg for widgets.
add_filter( 'use_widgets_blog_editor', '__return_false' );

add_action( 'wp_enqueue_scripts', function() {
    // Remove CSS on the front end.
    wp_dequeue_style( 'wp-block-library' );

    // Remove Gutenberg theme.
    wp_dequeue_style( 'wp-block-library-theme' );

    // Remove inline global CSS on the front end.
    wp_dequeue_style( 'global-styles' );
}, 20 );

<?php
add_filter( 'use_block_editor_for_post', function( $use_block_editor, $post ){
        if ( 'page' === $post->post_type ) {
                return false;
        }
        return $use_block_editor;
}, 10, 2 );