MySQL incluye cero filas cuando se usa COUNT con GROUP BY

Tendrás que usar un LEFT JOIN en lugar de un INNER JOIN . Comienza seleccionando todos los voucher_types y luego se unió a la izquierda para encontrar el conteo.

SELECT
  voucher_types.id AS voucher_type,
  IFNULL(vouchers_count.vouchers_remaining, 0) AS vouchers_remaining
FROM
  voucher_types
LEFT JOIN
  (
    SELECT
    v.type_id AS voucher_type,
    COUNT(v.id) AS vouchers_remaining
    FROM
    vouchers v
    WHERE
    v.sold = 0
    GROUP BY v.type_id
  ) AS vouchers_count
  ON vouchers_count.voucher_type = voucher_types.id