ẞ has been part of the official German orthography since June 2017.[1] It was however occasionally used before (and for example even by some editions of the influentialDuden dictionary entitledDER GROẞE DUDEN).
^Rat für deutsche Rechtschreibung (29 June 2017), “Amtliches Regelwerk der deutschen Rechtschreibung aktualisiert”, inPressemitteilung[1] (in German): “Sie lassen die Verwendung des Großbuchstabens „ẞ“ neben „SS“ zu, was insbesondere für die korrekte Schreibung von Eigennamen in Pässen und Ausweisen wichtig ist,[…] ―They permit the usage of the capital "ẞ" in addition to "SS", particularly important for the correct spelling of personal names in passports and ID cards.”
Capital ẞ on Wikipedia.Wikipedia