Storing things like issues in git is possible. But it makes it harder to make functionality on top like upvotes and issue boards. Not impossible for sure but it

Storing things like issues in git is possible. But it makes it harder to make functionality on top like upvotes and issue boards. Not impossible for sure but it slows down development. And it would only help with portably if rate limits are the problem. So far GitHub rate limits for logged in users are decent, we migrated 200k projects in 48 hours. Even when you store issues in repos the format would likely differ between different tools. If the format differs it is just as easy to write an importer that uses the API.