South Korean ETNews seems to have confirmation that the yet-to-be-announced Galaxy Z Fold 3 will be the first foldable with S Pen support. Pen support previously came to the Galaxy S series.
Samsung already hinted that the S Pen was not exclusively meant for the Galaxy Note series. The Galaxy S21 Ultra is also compatible and soon there will be support for the Galaxy Z Fold 3. This foldable phone has yet to be announced and is, in contrast to the Note and Ultra, equipped with completely different glass. Glass that is much more sensitive.