Because the T2 chip isn’t in older iPads, it doesn’t appear that the feature can be brought to older models in a software update.
9to5Mac found confirmation of the change in Apple’s latest Platform Security document:
“iPad models beginning in 2020 also feature the hardware microphone disconnect. When an MFI compliant case (including those sold by Apple) is attached to the iPad and closed, the microphone is disconnected in hardware, preventing microphone audio data being made available to any software—even with root or kernel privileges in iPadOS or in case the firmware is compromised.”
The wording of the entry implies that other, lower cost iPads released later in the year will include the feature, though this is unconfirmed.