The new S Pen Fold and S Pen Pro have a special retractable tip to protect the foldable screen.

Samsung Galaxy Z Fold3 is the first foldable phone with S Pen support, but apparently there are limits to the compatiblity. The company confirmed using old S Pens will damage the screen, and thats why it developed the S Pen Pro and the exclusive S Pen Fold Edition.
The two new styluses come with a tip, unlike any other S Pen. They are softer, and more importantly – retractable. When pressure is applied, the mechanism prevents damage to the softer foldable screen. If you apply excessive force with the traditional S Pens as seen in the Note series phones you may damage the screen even with the new 80% tougher protective sheet.
If you try to use an S Pen other than the two officially supported, the Fold3’s One UI will notify you that it’s incompatible and refuse to work with it.
The S Pen Fold Edition costs $50, while the S Pen Pro is $100. However, in many markets Samsung offers pre-order bundles with the S Pen Fold Edition and a case with a slot for it.