WordPress add support for page excerpts and show the excerpt metabox by default

gistfile1.txt

function dbt_add_page_excerpt_support() {
	add_post_type_support( 'page', 'excerpt' );
}
add_action( 'init', 'dbt_add_page_excerpt_support' );

function dbt_show_page_excerpt_metabox_by_default( $hidden, $screen ) {
	if ( 'page' == $screen->id ) {
		$hidden = array_flip( $hidden );
			unset( $hidden['postexcerpt'] );
		$hidden = array_flip( $hidden );
	}
  
	return $hidden;
}
add_filter( 'default_hidden_meta_boxes', 'dbt_show_page_excerpt_metabox_by_default', 10, 2 );