In case people are wondering, I’ve started Tracker #27282: Debian Buster support - Installer - Foreman. This is to run on Buster.
Note there are still some things to work out.
For example, https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=931572 makes proper support in forklift harder because the Vagrant image is still pre-release. We can work around this in our update role, but it’d be much easier to wait.
Puppetlabs hasn’t built Puppet for Buster but the native OS package is at least Puppet 5 so that allows some initial work.