How to Disable the Gutenberg Editor in WordPress

<?php
add_filter( 'allowed_block_types_all', 'cxc_allowed_block_types_call_back', 25, 2 );

function cxc_allowed_block_types_call_back( $allowed_blocks, $editor_context ) {

	return array(
		'core/image',
		'core/paragraph',
		'core/heading',
		'core/list',
		'core/list-item',
		'core/html'
	);

}
?>

<?php
add_filter( 'allowed_block_types_all', 'cxc_allowed_block_types_to_post_call_back', 25, 2 );
 
function cxc_allowed_block_types_to_post_call_back( $allowed_blocks, $editor_context ) {

	$allowed_blocks = array(
		'core/image',
		'core/paragraph',
		'core/heading',
		'core/list',
		'core/list-item',
		'core/html'
	);
 
 	// Show In Post Pages.
	if( 'post' === $editor_context->post->post_type ) {
		$allowed_blocks[] = 'core/shortcode';
	}
 
	return $allowed_blocks;
 
}
?>

<?php
$registered_block_slugs = array_keys( WP_Block_Type_Registry::get_instance()->get_all_registered() );

echo '<pre>' . print_r( $registered_block_slugs, true ) . '</pre>';

/*
Array
(
    [0] => core/archives
    [1] => core/avatar
    [2] => core/block
    [3] => core/calendar
    [4] => core/categories
	 
	 ... and so on
*/
?>

<?php
add_filter( 'allowed_block_types_all', 'cxc_blacklist_blocks_call_back' );

function cxc_blacklist_blocks_call_back( $allowed_blocks ) {
	// get all the registered blocks
	$blocks = WP_Block_Type_Registry::get_instance()->get_all_registered();

	// then disable some of them
	unset( $blocks[ 'core/archives' ] );
	unset( $blocks[ 'core/calendar' ] );

	// return the new list of allowed blocks
	return array_keys( $blocks );
	
}
?>