Disable Gutenberg globally or for specific post types for WordPress 5.0 and UP

thz_disable_gutenberg.php

<?php
/*
 * Disable Gutenberg globally 
 * use this if you prefer one-liner 
 * add_filter('use_block_editor_for_post', '__return_false');
 */
function _thz_filter_disable_block_editor(){
  return false;
}
add_filter( 'use_block_editor_for_post', '_thz_filter_disable_block_editor' );

/*
 * Disable Gutenberg for specific post types
 */
function _thz_filter_disable_block_editor_pt( $use_block_editor, $post_type ){
  if( 'page' == $post_type || 'post' == $post_type){
    $use_block_editor = false;
  }
  return $use_block_editor;
}
add_filter( 'use_block_editor_for_post_type', '_thz_filter_disable_block_editor_pt', 10, 2 );