Is grouping footers supported by the server side row model?... NO

No. The root cause here is that these row model handles the underlying data as an infinite stream of data, this makes very difficult any operation that implies a grand total.

If you are trying to implement this in your end, is maybe because your dataset is not that long, note that for datasets under 100K rows, the client side row model should be able to handle the data and this supports grouping footers

Note that we don't have any plans to add support for this since, as already mentioned, this row model treats the underlying data as an infinite stream of data.