Someone should write a local git+ssh service (+ email client) that simulates an upstream by hosting git repos locally, emailing all pushed commits/branches/tags to other team members, and keeping the hosted repos up-to-date via the emailed updates from others.

Bonus points for a local HTTP service that provides an optional GitHub-like experience in the browser, with comments/ branch reviews/etc. federated through emailed messages (and represented as commits on meta-repos).

With that data model, the primary remaining challenge would be setting up repos on a new machine -- perhaps BitTorrent could help :)

I feel like "communication" isn't descriptive enough. It's right, but eg Slack (or Microsoft Teams, or maybe both) isn't the right shape of communication.