SERVER-55689, we created a server parameter 'enableDefaultWriteConcernUpdatesForInitiate' that would allow us to update IDWC during reconfig. This is needed due to an optimization in our jstest infrastructure where we initiate replica sets by initiating a single node set, and then reconfig the whole membership set in. This server parameter should only ever be used by our testing infrastructure, but setting it to test-only in the IDL file caused test failures. Specifically, tests that manually disabled enableTestCommands failed because we couldn't set up a replica set test fixture, as we required setting the test-only 'enableDefaultWriteConcernUpdatesForInitiate' parameter in initiate.
As a result, we've omitted the test-only field from 'enableDefaultWriteConcernUpdatesForInitiate'. We should evaluate if leaving it as an undocumented server parameter would be sufficient.