There was a post in an unrelated thread remarking that an unnamed
writing system used the "at" sign (@) as a letter, and that optimal
encoding for that orthography was a non-starter.
A question as to whether that writing system was casing went unanswered,
but a kind list member offered some pointers privately.
The language in question is Koalib, which is spoken in the Sudan. It is
a casing script and the upper case form uses an upper case "A" with a
wrap around as in the lower case "@".
The current "solution" is for the users to use the P.U.A. for both upper
and lower case letters, and fonts such as Doulos SIL support that P.U.A.
encoding.
A Google search for "Koalib Unicode" finds the following:
Wikipedia:
https://en.wikipedia.org/wiki/Koalib_language
2004-08-25 Lorna A. Priest, Public Review Issue # 40
Revised Proposal to Encode...
http://www.unicode.org/review/pr-40-atsigns.pdf
2004-10-20 Doug Ewell, L2/04-365
The case against encoding the Koalib @-letters
http://unicode.org/L2/L2004/04365-pr40-ewell.pdf
2012-04-17 Karl Pentzlin, L2/12-116
"Capitalized Commercial At" proposal
http://unicode.org/L2/L2012/12116-capital-at.pdf
2018-12-26 Eduardo Marín Silva, L2/19-006
Proposal to encode...
http://www.unicode.org/L2/L2019/19006-capital-at.pdf
It's probably old-fashioned to say that technology should be forced to
accomodate people rather than the other way around. But it's good to
note that efforts are still being made on behalf of the users to make
progress towards U.C.S. inclusion.
