Something else I noticed while implementing CDRIVER-3400 is that _mongoc_collection_update_or_replace's error reporting for specifying arrayFilters when unsupported by the selected server appears to be redundant in light of existing logic in _mongoc_write_command_execute_idl (which is used for collation). That code dates back to c4af4aa for CDRIVER-2664.

We should see if setting the mongoc_write_command_t.flags.has_array_filters in _mongoc_collection_update_or_replace would allow us to delete the error reporting code in that function.