Make post excerpt as mandatory field with capability to set the lenght, if needed.

required-excerpt.php

// put this in functions.php or in your custom plugin or theme code.
$m = new Mandatory_Excerpt();
$m->add('post', 20 ); // will require all 'post' post types to have an excerpt at least 20 characters long

/**
 * Helper class which removes specified metaboxes from specified pages. It manages setting up the hooks
 * and calling them.
 * 
 * @author: Michael Fielding
 * 
 * Usage: 
 * 		$me = new Mandatory_Excerpt();
 *		$me->add('post', 20 )
 *		   ->add('my_custom_post', 44); // repeat as needed
 */
class Mandatory_Excerpt {
	private $min_lengths = [];

	// set up the hooks we'll need
	public function __construct() {
		add_filter('wp_insert_post_data', [$this,'do_mandatory_excerpt']);
		add_action('admin_notices', [$this,'do_admin_notice']);
	}
	
	public function add( $post_type, $minimum_excerpt_length = 0 ){
		$this->min_lengths[$post_type] = $minimum_excerpt_length;
		return $this;
	}

	// handler for wp_insert_post_data hook
	public function do_mandatory_excerpt( $data ) {

		$post_type = $data['post_type'];
		// if there's a minimum excerpt length set for this post type, check it
		if ( isset($this->min_lengths[$post_type]) ) {
			$min_length = $this->min_lengths[$post_type];
			$excerpt = $data['post_excerpt'];
			if ( empty($excerpt) || strlen($excerpt)<$min_length ) {
				// if user was trying to publish the post, show a warning
				if ($data['post_status'] === 'publish') {
					// add a filter to intercept the post-save redirect and show a message
					add_filter('redirect_post_location', [$this,'do_error_message_redirect'], 10, 2);
				}
				// ensure it's not published
				if ('deleted' !== $data['post_status'] && 'trash' !== $data['post_status']) {
					$data['post_status'] = 'draft';    
				}			
			}
		}
		return $data;
	}
	
	// intercept the after-save redirect and ensure a message will be shown on the following page
	public function do_error_message_redirect($location,$post_id) {
		remove_filter('redirect_post_location', [$this,'do_error_message_redirect'], 10, 2);
		$post_type = get_post_type( $post_id );
		if ( isset($this->min_lengths[$post_type]) ) {
			$min_length = $this->min_lengths[$post_type];
			
			// add a query variable to display our admin notice
			$location = add_query_arg('excerpt_not_long_enough', $min_length, $location);
			
			// remove the post saved message
			$location = remove_query_arg( 'message', $location );
		}
		
		return $location;
	}

	/**
	 * Show a warning or error that the excerpt is required, if it is not manually defined for something where it's required.
	 */
	public function do_admin_notice() {
		// show a warning or error if there is no custom excerpt on a type where there should be
		$screen = get_current_screen();
		
		if ( $screen != NULL && $screen->base == 'post' && isset($this->min_lengths[$screen->post_type]) ) {
			$min_length = $this->min_lengths[$screen->post_type];
			$post_id = $_GET['post'];
			
			if ( !has_excerpt($post_id) || strlen(get_the_excerpt($post_id)) < $min_length ) {
				$this->echo_admin_notice(['Cannot publish this until a hand-written Excerpt of at least ',$min_length,' characters is provided.'], 'warning');
			}
			
			// check if the excerpt not long enough message was requested by being put in the query string
			if (isset($_GET['excerpt_not_long_enough'])) {
				$this->echo_admin_notice(['Publication failed because the Excerpt wasn\'t ',$min_length,' characters or more.']);
			}
		}		
	}

	/**
	 * Show an admin message. Should be called in an admin_notices hook.
	 * @param string|array(string)  $text The notice to show. This is passed through _e() for translation.
	 * 		Can be an array, in which case each string is concatenated after passing thru _e(), which allows
	 * 		parameters to be embedded.
	 * @param string $class error|warning|success|info
	 * @param boolean $is_dismissable If true, then the notice has an X to dismiss it.
	 */
	private function echo_admin_notice( $text, $class = 'error', $is_dismissable = true ) {
		$dismissable = $is_dismissable ? 'is-dismissable' : '';
		echo "<div class=\"notice notice-$class $dismissable\"><p>";
		foreach( (array)$text as $t ) {
			_e($t);
		}
		echo( "</p></div>" );
	}
	
}