For the new iPad models, the front camera and FaceID sensor array have been relocated from the short-edge to the long-edge of the iPad - such that the camera and sensor array are now positioned top-centre of the screen (as seen with the iPad is positioned in Landscape orientation). As such, the camera/sensor array now occupy the internal cavity previously used for the second generation Pencil's inductive pairing/charging system.
To accommodate this change, the charging system for the Apple Pencil has been completely redesigned - moving the associated electronics to different cavity locations along the top edge of the iPad chassis. The new Apple Pencil Pro has been specifically designed to accommodate the new internal architecture.
Consequently, relocation of inductive coupling components within newer models of iPad and Pencil preclude use of the second generation Pencil with new models of iPad - and, by extension, new models of Apple Pencil are not back-compatible with older models of iPad.