From Simon:

we are now building with macOS 10.13 (High Sierra) as target (i.e. the oldest supported system), regular Apple Xcode/command line tools and GNU Fortran 8.2. R builds are running on macOS 10.15 (Catalina) with Xcode 11.4 using macOS 10.13 target. Packages are built on macOS 10.13 VMs with just Apple command line tools (this should make it easy to replicate the setup using Travis, for example). All 3rd party libraries that CRAN uses are available inhttp://mac.r-project.org/libs-4/

Source:https://stat.ethz.ch/pipermail/r-sig-mac/2020-April/013289.html

This means that macOS is no longer supporting a compiler compatible with OpenMP by default. So, custom compilations of RcppArmadillo will be required to enable OpenMP. I don't think that RcppArmadillo will be affected by this change as it already disables OpenMP whenwhich clang returns an Apple identifier.

Though, that should be verified on anr-devel build of macOS.