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

[nasser@merveilles.town]

@mcc I feel like I tried Arabic usernames and they didn't work

[thatdawnperson@toot.cat]

@mcc 왜 그렇게 되셨나요?

[mcc@mastodon.social]

@thatdawnperson I thiiiiink that the way they fit the antique Korean jamo into requires a really awkward hack that they just don't want these systems to have to deal with

[mcc@mastodon.social]

@thatdawnperson But seeing them lead with that just makes it seem oddly vindictive

[hearth@plush.city]

@mcc ...is there any reasoning given for this?? and for the latter two, those seem weird too
-F

[chrisamaphone@hci.social]

@mcc this was a subplot in Analog surely

[mcc@mastodon.social]

@chrisamaphone so remember, part of the revanchivist ideology in Analogue involved enforcement of writing in Hanja

[alilly@solarpunk.moe]

@mcc @Hearth @xgranade I'm guessing Q and R are disallowed to mitigate homoglyph attacks. Maybe Old Hangul too, which presumably contains some homoglyphs with modern Hangul.

[mcc@mastodon.social]

@alilly @Hearth @xgranade ohhhh wait that would make so much sense :O with the old jamo

[hearth@plush.city]

@mcc @alilly @xgranade that makes sense! homoglyph attacks are still possible with e.g. replacing latin o with greek ο or cyrillic о, though?

...unless that's what section Q is talking about, i don't know exactly what it means
-F

[alilly@solarpunk.moe]

@Hearth @xgranade @mcc … Damn, that might be a valid argument in favor of Han unification. How dare things I already made up my mind on have nuance I didn't consider?

[alilly@solarpunk.moe]

@mcc @Hearth @xgranade Yeah but that's much harder to do anything about, unless you want to ban modern speakers of languages written using Cyrillic from using names in their native language, which… don't do that.

[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).