The Decca FFSS PU heads from the earliest days of stereo in 1958 where designed to be entirely mono compatible. That is because the coils were not at 45/45 degrees to the record surface. One coil was horizontal as it would be in a mono cartridge and the other coil vertical. So it played a mono record like a mono cartridge. The downside was slightly less vertical compliance than horizontal compliance.
My Decca cartridge playing the first LP I ever bought, age 8 back in 1955.