The cords have been around longer than slot cards, it's just an adaptation of a keychain/safety strap for another use.

They should be durable, unless they come in contact with other types of plastics.

The same cords have been used for years on personal watercraft, outboard motors and snowmobiles as a safety kill switch attachment. You fall off, it shuts off the motor.

They look pretty inert to me, but if you put them in a box with fishing worms, there's no guarentee.