In February 2016, AT&T bumped the price of the plan from $30 to $35/month. Now it's going up another $5/month.
"Our Mobile Share Advantage plans and our AT&T Unlimited Plan provide several benefits that our legacy unlimited plan doesn’t," AT&T said in a statement. "If you have a legacy unlimited data plan, you can keep it; however, beginning in March 2017, it will increase by $5 per month."
AT&T has been trying to get users off its unlimited data and been fined by the FCC for misleading its customers about unlimited mobile data plans.