Print last modified date only on posts

You can check the post type with get_post_type(). You only want to display the content if the value is post:

function wpb_last_updated_date( $content ) {
    $u_time          = get_the_time( 'U' ); 
    $u_modified_time = get_the_modified_time( 'U' ); 

    if ( get_post_type() === 'post' ) {
        if ( $u_modified_time >= $u_time + 86400) { 
            $updated_date   = get_the_modified_time( 'F jS, Y' );
            $updated_time   = get_the_modified_time( 'h:i a' ); 
            $custom_content="<p class="last-updated">Last updated on ". $updated_date . ' at '. $updated_time .'</p>';
        }
    }

    $custom_content .= $content;

    return $custom_content;
}
add_filter( 'the_content', 'wpb_last_updated_date' );