Display image title and caption on posts and pages

you are trying to get the image title using get_the_title($image_id), but $image_id is not defined in your function. Instead, you should use the id attribute provided in the shortcode attributes.

add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 10, 3 );

function my_img_caption_shortcode( $output, $attr, $content ) {
    $attr = shortcode_atts( array(
        'id'      => '',
        'align'   => 'alignnone',
        'width'   => '',
        'caption' => ''
    ), $attr );

    if ( 1 > (int) $attr['width'] || empty( $attr['caption'] ) ) {
        return '';
    }

    // Get the image ID from the shortcode ID attribute
    $image_id = isset( $attr['id'] ) ? (int) $attr['id'] : 0;

    // Get the image title
    $image_title = get_post( $image_id )->post_title;

    if ( $attr['id'] ) {
        $attr['id'] = 'id="' . esc_attr( $attr['id'] ) . '" ';
    }

    return '<div ' . $attr['id']
    . 'class="wp-caption ' . esc_attr( $attr['align'] ) . '" '
    . 'style="max-width: ' . ( 10 + (int) $attr['width'] ) . 'px;">'
    . do_shortcode( $content )
    . '<p class="wp-caption-text">' . $image_title . ", " . $attr['caption'] . '</p>'
    . '</div>';
}