Missing Assyrian flag emoji on iPhone

Apple is not in charge of Emojis, but rather the Unicode Consortium is.

But the Unicode Consortium has already made a statement saying, other regional and geopolitical flags have no mechanism within the Unicode Standard. This includes historic flags as well as current flags who do not have a corresponding region code or subdivision code, i.e., Assyrian. Until such time as a general mechanism is proposed and accepted, proposals for flags in this category will not be considered by the Emoji Subcommittee or the UTC.