Related commits (at least):
https://github.com/mongodb/mongo/commit/4db60cf2
https://github.com/mongodb/mongo/commit/892aad0c2fd35985f12b052668f1e37195c17740#L37L1337

With these two commits, lines starting with LOG(LL_ERROR) are silently blackholed because LL_ERROR is an enum with value 4.

Also it means that MONGO_LOG() can't return a nullstream since it is called by error() and warning().