WordPress Subtitle Plugin

Add a subtitle input field to the post edit screen.
Register your custom post types with 'subtitle' support or add post type support via a filter:

add_action( 'init', 'prefix_add_subtitle_support' );

/**
 * Add a subtitle input field to the post and page admin screen.
 */
function prefix_add_subtitle_support() {

    $post_types = array( 'post', 'page' );

    foreach( $post_types as $post_type ) {
        add_post_type_support( $post_type, 'subtitle' );
    }
}

Use

The subtitle is saved to a custom field called subtitle.
The plugin does not add any template tags to retrieve it, use at your own convenience.

// When in the loop something along these lines might work:

if ( ! empty( $post->subtitle ) ) {
    echo esc_html( $post->subtitle );
}

// Or:

$subtitle = get_post_meta( $post->ID, 'subtitle', true );

if ( ! empty( $post->subtitle ) ) {

    printf(
        '<p class="entry-subtitle">%s</p>',
        esc_html( $post->subtitle )
    ); 
}