From speaking to QA, the issue is apparently that ARCore was stripped out of 4.18 when the Previews were released. From what I understand this was not intentional, which explains why it wasn’t mentioned. It has since been put back in but it is not currently in the latest preview (Preview 2) at the time of posting this. If you wish to use ARCore, you should be able to by downloading the latest 4.18 from Github.
Please let me know if you run into any other issues related to ARCore.