Illicit and illegitimate are essentially the same thing and illicit (or irregular) is the standard term for this sort of thing—they are valid sacramentally but illicity in terms of canon-law. Illegitimate means essentially the same, though it’s not commonly employed in Church-English.
You mean illicit like how the local parish where I live uses leavened bread for the Sacrament and admitted it was valid but illicit?