Please upgrade here. These earlier versions are no longer being updated and have security issues.
HackerOne users: Testing against this community violates our program's Terms of Service and will result in your bounty being denied.
Deleted User 'System'

Comments
You should update your install to the latest patch, currently 2.0.18.13: http://vanillaforums.org/get/vanilla-core-2.0.18.13
Once you are done with that, you don't have to worry. The next time the system user is needed, it will be re-created.
If you really want it back now for some reason, you can use the following code in a plugin to regenerate it:
Navigate to the dashboard once this is enabled and you should see your new System user.
Search first
Check out the Documentation! We are always looking for new content and pull requests.
Click on insightful, awesome, and funny reactions to thank community volunteers for their valuable posts.
nice, went the plugin route, thanks.