> "It's a US-specific rule"

And? Calling it a, "very odd colon rule" assumes that British English is somehow the correct one, which it isn't. The United States represents more than 70% of the world's native English speakers. If we have a rule, then it's not the "odd" one.

> "...only in certain style manuals."

And? The two most important style manuals used by millions of people to write billions of articles and papers.

> "Chicago, for example, does not capitalise after a colon."

Did mean "capitalize"? (I'm kidding of course, I know you people like to stick with your quaint antiquated ways of spelling.)

You might want to check that again:

https://www.chicagomanualofstyle.org/search.html?clause=colo...