The Release notes state:
" BREAKING: RStudio Connect will now verify on startup whether packages that have been specified in the Packages.External configuration setting are available in every detected version of R, and will fail to start if they are not available in a given version. To allow RStudio Connect to start with missing external packages, set the Packages.ExternalCheckIsFatal flag to false"
The actual config parameter name from the Admin Guide is ExternalsCheckIsFatal...with an "s".