This is what apple released in 2013:
"Previous versions of your apps are now available for re‑download by users who have already purchased them, allowing customers to use your apps with older devices which may no longer be supported by the current version of your app. If you do not wish to make these versions available, you can manage the availability of your apps' previous versions in the Rights and Pricing section of the Manage Your Apps module in iTunes Connect. Learn more."
If you go to an app in iTunes connect and select "Manage iCloud download settings for this app" you can see the option, which is setup to allow old versions to be used by default unless you check a box to say a particular old version was bad and shouldn't be used. The catch is that it appears this only applies for users who have previously installed the app, not for new users who may want it on an old device. This fits in with what one might expect from apple, not wanting owners of older devices to be able to get hold of new apps where support has been dropped.