I accidentally deleted /etc/ssh/ssh_config file.
I've tried purging and re-installing openssh-server and ssh but the file doesn't return.
How would I restore this file?
The file /etc/ssh/ssh_config is part of the package openssh-client. Therefore
sudo apt-get purge openssh-client
sudo apt-get install openssh-client
or
sudo apt-get install --reinstall openssh-client
Purging the package ssh makes no sense, it's only a meta-package for openssh-client and openssh-server and purging this package doesn't purge openssh-client and openssh-server.
The file /etc/ssh_config is a configuration file, which has special handling in dpkg, so it is preserved across package upgrades. If the file is deleted, this, too, is preserved, so merely reinstalling the package will not fix the problem.
Your choices are, to either
dpkg, giving it the --force-confnew option to overwrite configuration files.The latter option has the advantage of keeping other customized configuration files, renamed with a suffix of .dpkg-old.
To do this, you need a copy of the package file,
apt-get --download-only --reinstall install openssh-client
will download it to /var/cache/apt/archives, and
dpkg --force-confnew -i /var/cache/apt/archives/openssh-client_...
will install it (replace the ... with the version downloaded).