Consider a Mastodon/Fediverse handle, like @username@domain

[hearth@plush.city]

@alilly @mcc @xgranade yeah, i guess the difference with the hangul thing is that it's a safe assumption no one is using thsoe characters to write their names in modern times, which is not the case for greek or cyrillic
-F

[elizafox@social.treehouse.systems]

@mcc Oh! Yeah. It's because they don't have a well-defined canonical composition order, unlike modern Jamo, which do.

A weird bit of trivia: there is no composition for hanzi/kanji/hanja/chữ Hán characters (what many call "Chinese characters"). You can't just build one in Unicode. If you could, they'd also be in this list, for the same reason that Old Hangul Jamo are disallowed (which were only added because scholars needed them).