_mongoc_write_command_execute_idl raises an error if arrayFilters is used with an unacknowledged write concern. It's possible that this code is left over from when OP_UPDATE was still being used for executing unacknowledged updates.

This is more restrictive than what the CRUD spec requires for update options:

• For servers < 3.6, the driver MUST raise an error if the caller explicitly provides a value.
• For unacknowledged writes using OP_UPDATE, the driver MUST raise an error if the caller explicitly provides a value.

Documentation for FindOneAndUpdate is similar, but omits the reference to OP_UPDATE.

Note: the CRUD spec also does not have tests for using arrayFilters and an unacknowledged write concern, so that explains why this might have been missed.