WP Query where title begins with a specific letter

It’s not possible with WP_Query directly out of the box, but by using the posts_where filter to look for a custom argument on WP_Query, it is possible to add this functionality.

Assuming starts_with is the name of the argument we want to use, we can filter posts_where to add a WHERE clause limiting results to those that begin with the given value if starts_with has been set on the query:

function wpse_298888_posts_where( $where, $query ) {
    global $wpdb;

    $starts_with = esc_sql( $query->get( 'starts_with' ) );

    if ( $starts_with ) {
        $where .= " AND $wpdb->posts.post_title LIKE '$starts_with%'";
    }

    return $where;
}
add_filter( 'posts_where', 'wpse_298888_posts_where', 10, 2 );

With this filter added, we can query posts like this:

$query = new WP_Query( array(
    'starts_with' => 'M',
) );

That will return all posts beginning with “M”.

If you want to be able to filter the main query, you can also use this argument in pre_get_posts:

function wpse_298888_pre_get_posts( $query ) {
    if ( $query->is_main_query() ) {
        $query->set( 'starts_with', 'M' );
    }
}
add_action( 'pre_get_posts', 'wpse_298888_pre_get_posts' );