I've just rolled out the first install.sh (version 6.0.11) with support for Ubuntu 18.04 LTS. I've also created a new virtualmin-bionic repository with 18.04 LTS binary packages.
This should be considered a beta release. Please test your use case thoroughly before counting on it in production! We always expect issues to be found when new distributions are released, because things change in subtle ways. Versions of underlying software changes, paths may change, default options might change, etc. In my testing of the most common installation scenarios, it seems to work well, but Virtualmin has a gazillion options and manages a bazillion moving parts, so surely y'all will find something borked once you start using it.
If you run into any bugs, please let me know in the issue tracker.
Edit 7/10/2018: Updated to specifically mention the netplan issue, which is ongoing. So, still beta. Read the whole thread and links for more info on the status of that, if you want to install on 18.04 today. You must use the old network configuration scripts for now.