git annex transferkeys
This plumbing-level command is used to transfer data, by the assistant in git-annex version 8.20201127 and older. It is still included only to prevent breakage during upgrades.
It is a long-running process, which is fed instructions about the keys to transfer using an internal stdio protocol, which is intentionally not documented (as it may change at any time).
Joey Hess <email@example.com>