To help other people farm golden heroes, you mean? Conceding doesn't add anything towards your 500 ranked win count.
Farmers auto-concede to stay around the low ranks of ladder to win easier against bad players, accelerating the pace at which they earn their gold portrait.
i was wondering, there is some kind of penalty or ban for conceding at rank 20 to farm golden heroes?
Yes. The penalty is -1 star
why should there be a penalty?
To help other people farm golden heroes, you mean? Conceding doesn't add anything towards your 500 ranked win count.
Nope no penalty at all.
I heard that you stop getting gold for 3 wins if you concede too much on rank 20 or so. Not sure whether it is true.