These higher-level functions seemed useful at the time of implementation, but they were not needed by google-cloud-bigquery, pandas-gbq, or even experimental Dask (dask/dask#3121 (comment)). Instead, only the lower-level ReadRowsPage.to_arrow and ReadRowsPage.to_dataframe methods were used.

Deprecating these can allow us to deprecate the passing around of read_session fully because we won't have to worry about returning something sensible for empty streams.