It's been three years since Android's adoptable storage officially launched with Android Marshmallow, after a few months of testing in the Developer Preview. Many Android manufacturers didn't offer the feature immediately on their own phones, and until now, Samsung has been one of the holdouts. That may change with the upcoming update to Pie though.
User spenceboy98 noticed that his Note9 Pie beta had an interesting new screen when inserting a MicroSD card: it asks if you want to use it for extra phone storage or simply as an external storage. If you choose the former, ie the adoptable storage option, it will be counted with your internal storage and can be used for apps, games, etc. However, it seems that the process doesn't complete successfully and an error stops it midway.
Although this isn't live yet, it's close enough to make us believe that Samsung is working on rolling out adoptable storage on its devices with the Pie update. You may not need it, but for those who have a lot of stuff saved on their phones, this will be a welcomed improvement.
- GSM Arena