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There are lot of tools to migrate them. Example, github.com/babel/babel has migrated their issues to phabricator.babeljs.io

True, but thanks to the API and their relatively simple structure it's reasonably easy to at least copy their contents as well. Linking them correctly to user accounts on a new platform is probably the biggest issue.

Users with public SSH keys would be easy: https://github.com/JohnMaguire.keys

When a new user signs up, ask them for their existing Github username and to upload an SSH key that matches.

I am pretty sure that a potential competitor will use GitHub's APIs to import issues as well.

+1

You say that like it is a bad thing...