Shifter support comments through 3rd party services like Disqus. It's a commenting platform that offers filtering tools for spam, comment support for multiple providers and CMS platforms and it works great on Shifter!
While migrating your site Disqus comments seem to vanish. This could be related to how their system works and they do offer a migration tool to resolve it.
If you are migrating to Shifter and your blog currently uses Disqus check out their Migration Tools for your particular migration needs and fix.
For more info on the topic of migrating comments with Disqus we found these articles helpful.