I did not get a letter. I bought bitcoins from Coinbase many years ago, but never sold to Coinbase, yet. Also bought bitcoins from Circle which was a much better place to buy from as they accepted CC with ease. Had I known Circle would stop operations, I'd have loaded up as much I as I could. Localbitcoins was another place I bought bitcoins from.
Bitcoin is anonymous, there are no names to the wallet addresses, but the exchanges are not, so selling a few bitcoins at Coinbase would trigger the $20K threshold, and most likely get the letter in the mail.
Circle (partly owned by Goldman Sachs) bought Poloniex for $400M, recently announced they are moving operations outside of the US. Bittrex and Poloniex were doing KYC since a year or so ago and I stopped using them as my accounts with them were under fake names, so would have to create new accounts with my real name. Binance announced they need to do KYC for US-based IP addresses. Abra an app-based cryptocurrencies wallet and exchange platform is suspending services for all US residents.
I have a long post on the bitcoin price thread on the uncertainties faced by US cryptocurrencies investors/participants. I believe that the US is moving to regulate bitcoin and cryptocurrencies and it's wise for all involved to follow the legal development and to comply. If I get a letter, I'd be meeting with the tax accountant on the next available appointment.