ARCore first came onto the scene in August of last year with support for the Pixel and Galaxy S8, with more phones added late last month to the official compatibility list. The Galaxy S9 and S9+, which were announced shortly after that latest update, aren't on that list yet. That'll change "in the coming weeks," though.
According to the ARCore Device Support Requests thread on GitHub, "Support for the S9 and S9+ will be added in the next release, which should be released in the coming weeks." It's a generic estimate, but at least it shows that progress is being made on the effort.
ARCore is now being used in over 85 apps, with major additions lately including IKEA Place, eBay, and Google's 'Just a Line.' It's nice to see artificial reality trickling down from dedicated AR phones like the Lenovo Phab2 Pro and ASUS ZenFone Pro to more mainstream ones.
It's also worth noting that ARCore 1.2 seems to have enabled support for the LG V30, which still isn't listed as compatible (unless the "V35" in the GitHub list refers to it).