Disable Gutenberg only for Posts

// =========================================================================
// DISABLE GUTENBERG FOR POSTS
// =========================================================================
function wpflames_disable_gutenberg($is_enabled, $post_type) {
	
	if ($post_type === 'post') return false; // it could be any kind of custom post type
	
	return $is_enabled;
	
}
add_filter('use_block_editor_for_post_type', 'wpflames_disable_gutenberg', 10, 2);