We have a custom build activity, which performs the execution of our build. We use a custom activity, because the compilers we use are very special and the system is quite old a
mtbwt:BuildTrackingParticipant.Importance="Low"/"None" only controls the call (I guess). Instead use mtbwt:BuildTrackingParticipant.TrackingOption="None", which seems to get completely rid of all tracking messages in the call.