Everywhere the Slavonic liturgy took root, Glagolitic was replaced by Cyrillic within a few generations, with one substantial exception. On the eastern Adriatic coast and the islands off it, in the Croatian lands, the script was not replaced. It was reshaped into an angular hand that looks quite unlike the rounded letters of the Bulgarian and Macedonian codices, and it went on being written for centuries, in liturgical books, in charters, in notarial records and in inscriptions cut in stone.
That tradition produced the most famous Glagolitic monument, a stone slab from the island of Krk carrying a long inscription in the angular hand, and it produced the first book ever printed in Croatian, a missal set in Glagolitic types in 1483, only a few decades after printing itself began. Glagolitic liturgy in Church Slavonic continued in parts of Dalmatia into the modern period.
A letter like this one belongs to that history rather than to the mission. It records a sound the Croatian scribes wanted a sign for, in a hand those scribes developed, in books they used, long after the alphabet's makers were dead. Counting it as a letter of the alphabet Constantine devised would be an error of a common kind: mistaking the whole life of a script for its origin.