Potential for Other Missing Features
Samsung sometimes holds software features back from older phones due to hardware limitations. It also likes to keep some software features exclusive for a period to help sell the flagships.
For instance the Galaxy S9 Android Pie update includes two Galaxy Note 9 features (Scene Optimizer and Flaw Detection), but it's unclear if those features will make it over to the Galaxy Note 8 and Galaxy S8 in the official version.
The Galaxy Note 9 Android Pie update doesn't have any huge changes for the device's S Pen on board. This doesn't mean there won't be changes on board, but users who rely on the S Pen should start tempering their expectations.
Samsung is also planning to launch its Galaxy 10 in early 2019. The Galaxy S10 will likely come with additional tweaks to Android Pie and Samsung's Experience UI.
If Samsung decides to bring some of the Galaxy S10's software features to the Galaxy Note 8, it will likely take months for that to happen.