AFAICT ”a shambles” is correct usage. See https://brians.wsu.edu/2016/05/24/in-shambles-a-shambles/
"SHA-1 is a slaughterhouse"
"SHA-1 is a shambles"
Beyond that prefixing your comment by "downvoted" is frankly silly and only serves to derail the conversation IMO.
You are of course right that "a shambles" is perfectly fine British English, but that's beside the point.
Not sure how it would be measured, but I think a /r/dataisbeautiful map would be interesting to see.
I'd think "in practice" would mean evidence of contemporary usage.