I believe it means you can pick a wildcard from any of the previous jams and implement it into your game, but you'll obviously have to disclose which card you picked.

I may be wrong, but this is my interpretation

I suppose it means exactly what it says.

1) Use the mechanics of old wildcard(s) in your game   (implement their mechanics)
Or
2) Show old wildcard(s) in your game (show off the card(s) visually)

festmester is correct!
Full list of old wildcards are available here https://godotwildjam.com/past-jams/
And images for wildcards are in corresponding folders here https://drive.google.com/drive/folders/13sDwqrs89RnZDNIH5FqNy8qz0HZtNd0C?usp=sha...