Due to a refactor during the 3.4 development cycle we no longer accept dotted field names inside a $group stage's _id field. So this pipeline no longer works, and returns an error of "FieldPath field names may not contain '.'"

[{$group: {_id: {"name.subname": "$foo", bar: "$bar"}, count: {$sum: 1}}}]

We should add a note about this to our compatibility notes.