Some reddit users (1,2, 3) have reported receiving a letter from U.S. Bank informing them that their Cash+ card would be converted to the Altitude Go card. Previously U.S. Bank had sent out letters offering cardholders that hadn’t used the card recently the option to convert, but not it seems this is a forced conversion. The Cash+ is still accepting new applications, so this doesn’t seem to be a case of the card being discontinued.
Source: doctorofcredit.com