The STD Bus is a relic of a past, yet still relevant today when it comes to building designs which require a pluggable board design.
Consider a chassis which is populated with say, 2 x Raspberry Pi for high level logic and 4 x Arduino or STM32 controllers for low-level control. The passive backplane provides power and common communication paths. The prototype boards have footprints at the edge for common connectors such as screw terminals and DB9/25, and accomodates a Pi or Arduino board/shield in the middle.
If you are not using the chassis with other STD or STD32 compatible cards out there, the common data lines could be used for any other signalling as long as the noise properties are acceptable. Unless you are maintaining an old system or a retro enthusiast, there is usually little reason to work with old boards anyway.
While the availability of base/backplane units are common on eBay, the cost of prototyping cards are rather prohibitive, making it reasonable to do a fabrication run of your own design.
Fabricated at 1.6mm thickness, 1oz copper.
Removal from slots is hard without ejection clips that typically come with commercial boards. It is possible to 3D-print them, but a zip-tie functioning as a pull-ring at each corner of the board would suffice.
Insertion into the slots was a bit restrictive, possibly due to the lack of a chamfered edge at the unplated fingers. Well, this is not really a dealbreaker for a prototype board though.