Apple calls vintage the devices that were sold five to seven years ago. These are now only repaired subject to hardware availability – which is usually given. Apple does not repair devices that are seven years old and older, but the chances are good for most of the five to seven year old devices.
In addition to the iMac 5K from mid-2015, the end of 2014 model and its predecessors from late 2013 to mid-2014 were also added to the list. It’s a bit ironic that the old iMacs are already classified as vintage when the same design is still sold – including Intel processors.
If a product is on the vintage list, or even on the obsolete list after seven years, this does not necessarily mean that the software support will stop immediately: on average this is seven years with new macOS versions. This is usually followed by two years of security updates.