Since the Gutenberg editor has been added to WordPress and started using as the default content editor, many developers and ordinary users still prefer the old editor.
In order to turn off the new editor and get the old one, you will need to add a few rows of code into your function.php:
add_filter('use_block_editor_for_post', '__return_false', 10);
add_filter('use_block_editor_for_post_type', '__return_false', 10);
Also, in the newer versions of WordPress, the Gutenberg editor is turned on for the widget dashboard. As the editor for the content, the Gutenberg widget editor is not habitual for the regular users. Fortunately, you can turn off it in the same way, so you need merely add a few lines of code to your function.php:
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false', 10); add_filter( 'use_widgets_block_editor', '__return_false', 10);
After these changes in your function.php, you will get the old interface of the content editor for posts and the old interface to manage widgets.