Samsung recently confirmed that the Galaxy Note 9, Galaxy S9, and Galaxy S9+ were in the queue to receive One UI 2.5, however, it was not mentioned at the time whether the devices would receive the full feature set of the skin or if they would end up with a trimmed-down version. It now appears it will be the latter.
According to a post made by a Samsung representative in Korea, hardware constraints inherent to the Galaxy Note 9 and Galaxy S9 series mean that some functions of One UI 2.5 will have to be left out so that the smartphones from 2018 can at least sport the latest update, even in a cut-down form. Memory restrictions are also being considered as part of a review process for releasing the skin for these older flagships.
However, the fact that Samsung is currently putting together the One UI 2.5 build for the Galaxy Note 9, Galaxy S9, and Galaxy S9+ indicates that an update should be coming soon. Once the review phase is complete, a fully stable version of the Android-based skin should be pushed out, with the South Korean OEM skipping a beta-update testing phase.