There was a recent change (#1831) that caused rmarkdown to fail on R versions older than 3.6.

This issue could have been caught before being released to CRAN if rmarkdown was being checked on older R versions using CI.

The tidyverse team does this to ensure our packages support the last 5 minor R releases, which is now R 3.3, 3.4, 3.5, 3.6 and 4.0

It would be great if we could do rountily have similar checks for the knitr / rmarkdown stack, to ensure we don't lose compatibility with these critical packages accidently in the future.