The Airbnb superhost designation is sure to get you one thing: A $100 voucher which can be used on booking an Airbnb. It has to be used within a year and if you book a place for less than $100 you lose the leftover but other than that there is no catch. Since I travel using Airbnb that is real money to me. Aside from that SH doesn’t get you anything.
Given Airbnb’s emphasis on using the internet I wouldn’t be surprised if snail mail to their corporate offices goes directly into the recycling bin.
I’d be shocked if they answer letters.