I moved this from the Google Home forum to the Tab S forum.
I think Marshmallow still supports most major apps out there, but will probably start losing support over the next year or so. It doesn't mean the tablet will be completely unusable, but you'll probably start to notice more and more apps no longer being compatible. In that case, it'd be important not to uninstall any apps that are important to you, because if those apps have been updated to the point where they require Nougat or above, you won't be able to reinstall them. As long as you have the older version still on your tablet, it'll probably remain functional.