In MongoDB 4.0 and earlier, we explicitly ban readConcern level "majority" for aggregation pipelines containing $out.

    auto readConcernLevel = repl::ReadConcernArgs::get(expCtx->opCtx).getLevel();
    uassert(ErrorCodes::InvalidOptions,
            "$out cannot be used with a 'majority' read concern level",
            readConcernLevel != repl::ReadConcernLevel::kMajorityReadConcern);

However, it does allow readConcern level "linearizable". As linearizability is a property of a single document, we will ban a $out aggregation with readConcern level "linearizable", as it isn't clear what semantics will be offered to our users.

This is a backwards-breaking change in 4.2; however, we will allow support for readConcern level "majority" in $out pipelines in SERVER-20445.