https://wccftech.com/s-pen-galaxy-note-9-not-compatible-with-note-8/

The S Pen belonging to the upcoming Galaxy Note 9 is expected to come with a ton of features, casting a shadow over the previous-generation stylus’ list of functions. However, users would assume that the Galaxy Note 8 would also be compatible with the latest S Pen, negating the need for users to purchase a brand new Galaxy Note flagship in order to take advantage of those upgrades. Unfortunately, that isn’t going to be the case, because a new rumor claims that if you’re in possession of a Note 8, chances are the upcoming accessory will work just as a regular accessory and nothing more.

It does make sense for the Galaxy Note 9’s S Pen to just be compatible with the upcoming flagship; Samsung could have outfitted the Note 9 with some special internals that will allow users to seamlessly pair the smartphone and the stylus. According to Ice Universe, he does not believe that the Galaxy Note 8 will be compatible with the S Pen that will come shipped with the Galaxy Note 9.