The adoptable storage feature was introduced with Android Marshmallow a few years ago and it made sense for devices with low built-in storage since the microSD card acted as an additional space not just for your media but your apps as well. Unfortunately, Samsung chose not to adopt that feature on the premise that only low-storage, low-end devices would benefit. It all changes with Android Pie, though.
According to post in XDA Developers, the adoptable storage function will be enabled with the upcoming Pie update for the Galaxy S9-series and the Note9. The feature has been found in the leaked Galaxy Note9 firmware, but it doesn't work properly for now.
Although a useful feature, it doesn't take away the fact that Samsung is pretty late on this one as most Galaxy devices have at least 32GB of internal storage, which is enough for your apps while the microSD card will take care of your media library.
One of the pictures show there's an option to keep the sd card for files only, hence it would work as it always has been on previous Samsung devices