But I have come across other old spellings, like "j" used in diphthongs in place of "i", like in "jeri" (old spelling for "ieri", "yesterday") or in "naja" (old word for compulsory military service time). So it must be even older/rarer than that, and I would still say "j" it's not an Italian letter because nobody uses it exept to write "Jesolo" but that's a name, not a regular word.

Fun fact: because of the old usage of "j" some text to speech are "broken". The one on railway speakers always reads "RailJet" as "Railiet" which sounds funny.