The Apple Super Serial card did come with a metal back plate for the DB25 connector, which was clamped to the interior conductive coating on the plastic cases of later revisions of the Apple II+ and the Apple IIe.
IIRC, circa 1981, the Apple II+ with Disk drives and a Super Serial Card plugged in was tested for compliance with FCC Part 15 RFI/EMI regulations, when used with a shielded serial cable (which many hobbyists likely didn’t use).
So expansion slots were not fundamentally incompatible. But it did take a few years before Apple made that available in revisions of the Apple II that included coated plastic, conductive seals, and back plates for the expansion board connectors.