For reasons unknown the Vim project decided to switch mouse support *on* by default, and the Debian package maintainer decided to just flow that downstream in Deb 10. It was a… contentious change. I have no idea what other distros are doing. You’ll know you have this if you go to select text in your terminal window and suddenly Vim is in “visual” mode.
Supposedly the Vim project can’t change the default (again) because “people are used to it” (uhhhh).
The issue is that while there is an include from /etc/vim/vimrc to include /etc/vim/vimrc.local but either creating this file apparently either disables every other default option completely (turning off, for example, syntax highlighting) or the defaults will load after it (!?) if the user doesn’t have their own vimrc and override your changes.
Mostly I’m happy with the defaults, I usually just want to change this one thing without having to set up a ~/.vimrc in EVERY HOME DIRECTORY
Anyway, here, as root:
cat > /etc/vim/vimrc.local <<EOF
" Include the defaults.vim so that the existence of this file doesn't stop all the defaults loading completely
source \$VIMRUNTIME/defaults.vim
" Prevent the defaults from loading again later and overriding our changes below
let skip_defaults_vim = 1
" Here's where to unset/alter stuff you do/don't want
set mouse=
EOF