Greenbone provides all parts of our core as free software (called GSE/GVM) and there are no plans to change that ever. Of course we are using some small patches in our products to adjust some configs e.g. for branding.
Therefore it is still possible to create a master/slave setup with GVM 11 but you have to dig through the source code. Currently no public configuration documentation for master/slave exists. So you either should buy a Greenbone product, use GVM 10, use GVM 11 with a GMP sensor or dig through the source codes for using an OSP sensor.