Even though the mapReduce command is deprecated, customers still rely on it, and there are no plans to remove it (details on this are in the comments). With legacy timeseries mapReduce is not supported because mapReduce cannot run on a view. However, with viewless timeseries we do not raise an error, but we also don't do timeseries translations on the pipeline. We have two paths forward

(a) We continue to restrict mapReduce from running on timeseries collections.

(b) We can add timeseries translation logic to mapReduce so it can correctly run on timeseries collections. 

I think for the viewless timeseries initiative we can go with option a, since the goal is to have feature parity with legacy timeseries and it will be easier than option b. If option B is not that much more work we might consider expanding support.