This may not be trivial to implement, but from a previous discussion it may be useful.
Users may not be entirely aware that when they use the Mapper to write/update data that any null fields on a mapped object will cause the Mapper to write a 'null' value creating a tombstone. More often than not, this behavior is not desired. In 2.1.7 the driver added saveNullFields to alter this behavior, but not writing nulls was not made the default for compatibility reasons.
If possible, it would be user friendly to emit a warning after X null fields written that null values are being written from the mapper, and that the user might want to consider looking into saveNullFields if this behavior is not desired. We would likely want to use a specific logger name for this so users could suppress this behavior if they wanted.