How many versions of R to support at a time?

Our current policy has been to support 2 versions of R at the same time, typically the first patch release from both the current minor release and previous minor release (e.g. 3.4.1 and 3.5.1), until supporting the older version becomes too cumbersome (e.g. why we stopped supporting 3.4.1).

We recently built R 3.6.1 for conda-forge. As we starting building R packages for R 3.6.1, should we continue to support R 3.5.1?

cc: @conda-forge/r @conda-forge/core
xref: https://github.com/conda-forge/conda-forge-pinning-feedstock/pull/245, https://github.com/regro/cf-scripts/pull/540, https://github.com/conda-forge/r-base-feedstock/pull/89