the answer is: secret20 which has been there since Sept 20, 2010

I also found another 6 cards that have been growing mold since 2010

that's longer than i have been in colors

I'm just glad that I don't have anything from prejoin/the first month

apparently that Sept I got a lot of useless cards because 4 out of the 7 old cards that I found were received that month

including 2 different spy cards... I guess no one wants that deck

I was doing this search to see what I could get rid of for the anniversary event

but now I'm tempted to keep at least secret20 around to see how long it will take until someone actually needs it.

you should totally keep it now, it's basically an ancient artefact now