The BX10s come for free with most dedicated root servers, and are meant to be used as free backup space. They will continue to be free. They are not visible anymore on the website because they are only available to customers who already have dedicated root servers.
Customers who already have free BX10s can continue to use them for free. If they would like to upgrade to a BX11 or something else, though, we ask them to pay the monthly fee. --Katie
Current users weren't automatically updated to the new plan, so for them it's a big storage upgrade at no extra cost.