/usr/share/vim/vim80/syntax/groff.vim is in vim-runtime 2:8.0.1453-1ubuntu1.
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 8 9 10 | " VIM syntax file
" Language: groff
" Maintainer: Alejandro López-Valencia <dradul@yahoo.com>
" URL: http://dradul.tripod.com/vim
" Last Change: 2003-05-08-12:41:13 GMT-5.
" This uses the nroff.vim syntax file.
let b:main_syntax = "nroff"
let b:nroff_is_groff = 1
runtime! syntax/nroff.vim
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