Potential for Missing Android Pie Features
Samsung sometimes holds features back from older devices due to hardware limitations. It also likes to keep some software features exclusive for a period to help sell its flagship models.
We expect older Galaxy Tab models to get a robust version of Android Pie, but the Galaxy Tab S4 will probably get the best version of the operating system.
If Samsung decides to bring the latest software features from Pie and the Galaxy Tab S4 to older Galaxy Tab models like the Galaxy Tab S3, it probably won't happen overnight. And it might not happen at all.
Third-party developers will likely bring missing features to older models down the road, but installing them will require time, effort, and a skill-set that some users might not have. One other note.
The Galaxy Note 9's Android Pie update was light on S Pen features and upgrades so don't expect any major upgrades for the Galaxy Tab's S Pen either.