https://developer.apple.com/documentation/macos_release_notes/macos_10_15_beta_release_notes
Didn’t check full info about this beta release note, but it seems that in newer operating system from apple some scripting languages won’t be included by default. We’re using posix tools because they are available on most unixlike oses, but it seems that now things may change a bit
There’s not much you can do in this situation, besides either adding checks to plugins - which should be done anyway due to differing versions, binary not in PATH, etc. - or bundle the dependencies & binaries along with the plugin (not very feasible).
Look at the versions of bash that ship with macOS or the default Python version on Ubuntu systems. The language-agnostic approach is super powerful, but this is just one of the things that have to be considered - perhaps a more standard way of checking for dependencies added to core would standardise and streamline this?