Disable Gutenberg Editor in WordPress

How can I disable Gutenberg without using a plugin ?

To disable Gutenberg and keep the original editor, simply add this 2 lines in functions.php or your plugin:

<?php
// disable Gutenberg for posts
add_filter('use_block_editor_for_post', '__return_false', 10);
// disable Gutenberg for post types
add_filter('use_block_editor_for_post_type', '__return_false', 10);
?>