/usr/share/vim/addons/syntax/notmuch-compose.vim is in notmuch-vim 0.12-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | runtime! syntax/mail.vim
syntax region nmComposeHelp contains=nmComposeHelpLine start='^Notmuch-Help:\%1l' end='^\(Notmuch-Help:\)\@!'
syntax match nmComposeHelpLine /Notmuch-Help:/ contained
highlight link nmComposeHelp Include
highlight link nmComposeHelpLine Error
|