Pandoc 2.0 contains a large number of changes, some of which change behavior in important ways. While we don't bundle 2.0 yet, we do use the system version of Pandoc over our own in some configurations, so need to ensure compatibility with the new version.
@jjallaire pointed out that we may need to adjust our self-contained HTML generation function here so that it works with both Pandoc 1 and Pandoc 2:
Since Pandoc 2 already exists in the wild, we should actually make this change for 1.1 patch (changed milestone). The work that should happen in 1.2 is updating the internal copy of Pandoc to Pandoc 2, which is now tracked in #1807.
It is possible to set RSTUDIO_PANDOC to Pandoc 2, even in RStudio 1.1, but it's probably not common. Compare with find_pandoc in the rmarkdown package, which will use a new pandoc in preference to RSTUDIO_PANDOC if it finds one.