your POST card is fine, they just re-purpose "3.3V" as "-5V" when used in a ISA slot.
Unfixable 486 boards are quite rare and I've dealt with a lot.
sadly I can't agree with you. What would you do with the boards with lots of battery damage and the boards which simply output "----" on POST cards like this one ? I even have boards which used to work fine and had no physical damage, come back few months later and it does nothing anymore.
Now @Bernkastel7734 if your board managed to output a POST code at some point there's still hope in my opinion. Maybe there's a cracked solder joint somewhere for example ? Check the simm slots for bent pins. Try to press on the chipset with one of your finger too just in case the chipset has broken solder joints.