FindAndModify operations contains suspicious code to throw in case of hist was provided for unack writes. Originally it was:

Feature.HintForFindAndModifyFeature.DriverMustThrowIfNotSupported(wireVersion) || (WriteConcern != null && !WriteConcern.IsAcknowledged)

Note OR in the condition.

But accordingly to the CRUD spec:

or unacknowledged writes and servers < 4.4, the driver MUST raise an error if the caller explicitly provides a value. 

So most likely it should be AND.

There are spec tests for this requirement that needs to be synced as well.