Is someone really going to struggle to find the Iris data-set or whatever if they already know the name of it? The current wording of the off-topic reason is
Questions about obtaining specific datasets are off-topic because they are too specialized and narrow in scope.
which I read as allowing requests for non-specific data-sets that can be used to make some statistical point.
So IMO
Request for US macroeconomic indicators by year—off-topic.
Request for dataset to test regression procedures with nearly collinear predictors—on-topic.
Request for Longley data—lack of research.