They obviously never caught on because they've since disappeared entirely. However they're just the thing I need for a project.
Bit of a long shot but I'm sure someone will have bought some and never gotten around to fitting them - if you have a kit in a box somewhere I'd pay the original price for them.
A few years ago a company called Retrofitz made plates for installing SPD cleats in normal shoes.
https://bikerumor.com/2013/02/21/retrofitz-makes-any-shoe-clipless-offers-diy-cleat-system/
They obviously never caught on because they've since disappeared entirely. However they're just the thing I need for a project.
Bit of a long shot but I'm sure someone will have bought some and never gotten around to fitting them - if you have a kit in a box somewhere I'd pay the original price for them.