The router commands create and createIndex became BasicCommandWithRequestParser to be able to attach the WriteConcernError field to their replies.

Keeping them as a TypedCommand was not an option because both idl replies were marked as strict (here and here); hence, it was not possible to add the WriteConcernError field to the idlzed reply.

However, we removed the strictness of both replies under SERVER-100938, so it'll be possible to add the WriteConcernError as a field of CreateCommandReply and CreateIndexesReply once v9.0 branches out. With that, we'll also be able to make both commands TypedCommand again.