vim: Why does the sh colour scheme highlight `$(...)` as an error?

For example:

I was under the impression开发者_StackOverflow中文版 that $(...) was recommended over `` because it's easier to nest.

Because your .vimrc file does not contain

let g:is_posix = 1

See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=552108

You are correct that $( ) is recommended over backticks as the latter has been depreciated. You're also correct that $( ) is POSIX shell. However, to the best of my knowledge, $( ) was added to the POSIX spec in 2004 and assuming you have the latest version of vim, it is quite likely that the sh.vim syntax highlight file has not been updated to reflect this change in the POSIX shell standard.