How retrieve or recreate a delete file from the /etc/ssh directory

First: if you're going to be messing around with system files like this, you really should have a backup. Actually, you should always have a backup of anything you don't want to lose, but usually that's just your user files. But if you're messing with the OS, you should probably have a backup of that, too, and probably a bootable installer drive in case things go seriously sideways (there's recovery mode, but it's hard to control what macOS version you get with that).

As for the files, there are actually 3 categories:

1. Public/private keys: These are generated automatically when the ssh service ("Remote Login" in the Sharing pane of System Preferences) is started. They're not needed if you only run the ssh client (and won't even exist if you never started the server component). If they're deleted, just turn the ssh service off and back on.

   However, if they're recreated, any client that's connected before, and connects again after recreation, will notice that they've changed and give a warning, because the server might be an imposter. You should avoid this if possible.

   (When I'm upgrading/replacing a server, I always save these and copy them to the new config, in order to avoid clients getting spurious warnings. I don't want to train users to ignore security warnings.)

2. Configuration files: as far as I know, these are from the standard OpenSSH distribution, and can be downloaded from opensource.apple.com. The versions for OS X 10.11.6 are here

   But there's a caveat here as well: these may get changed in things like security updates, so the actual version you have might be newer than what originally came with OS X 10.11.6, and there might be good reasons they were updated. You can check which updates included these files with e.g. pkgutil --file-info /private/etc/ssh/ssh_config, but I don't know of a good way to find out what version of the file was included with the listed updates. Maybe just reinstall the latest security update for your OS version?

3. The moduli file is sort of a hybrid of the above. It's part of the standard install, and available with the other config files, but you can generate your own with the openssh-keygen command if you want to. It's actually slightly more secure to generate your own, but kind of a hassle for minimal increase in security.