Attempting to use the bulkWrite command on 7.x servers yields the following error:

CommandError

{ code: 40415, code_name: "Location40415", message: "BSON field 'bulkWrite.errorsOnly' is an unknown field.", topology_version: None }

Pre-7.0 servers return this error instead:

CommandError

{ code: 59, code_name: "CommandNotFound", message: "no such command: 'bulkWrite'", topology_version: None }

Since drivers generally forward these kinds of errors directly to users, it would be more clear for 7.x to return the "CommandNotFound" error. The "unknown field" message may not make it clear to users that their server does not support the feature they're attempting to use.