The policy was always to have two active "major" releases on active engineering support. When a new major release came out, the one from two releases prior would go EOL 90 days later.
However - every maintenance release and EBF for PB between 11.0 and 12.5.2 had to be pulled because they included the Telerik web controls. SAP decided not to renew that OEM agreement, so we were legally restricted from distributing any release that included those 3rd-party components.
12.5.2 was the first PB release with the WebForms target removed (and the Telerik controls). That's why you can only get 12.5.2 and 12.6 right now.
-Paul-