Good Morning Michael,
Generally Maximizer Software support the current version and two versions back, at the moment this is 10.5, 10 and 9.5. Maximizer Professional 9 (the single user and small workgroup version) is still supported as well.
When a new version is released there is generally a window of opportunity for users of the older versions to upgrade before support for their version is removed and they are no longer able to purchase licenses.