The heartbeat version field was added in v3.6 to stop communication with v3.4 nodes when FCV 3.6 is set. v3.6 do so by adding a "hbv" field to heartbeat requests, which v3.4 nodes receive and validate that no unexpected fields are present in the request – v3.4 does not expect 'hbv', so it fails the command.

SERVER-32636 will close outgoing connections to old binary nodes on FCV upgrade. v3.8 (like v3.6) nodes will be unable to establish connections with older nodes once FCV is upgraded. Closing outgoing connections on FCV upgrade closes the loophole of v3.8 and v3.6 nodes continuing to communicate via previously established connections, and the heartbeat version check will be no longer needed.

See SERVER-31631 for history of heartbeat version.