Answer:
Most newer models are supported 2-3 weeks after the public release, sometimes even before that. Some older models that are not supported are being researched for possible support in the future, although this may or may not happen depending on the printer and technical details involved.
Some printers are unsupported and will most likely stay unsupported. The reasons for this may include issues with intellectual property, technical issues, or limited production numbers. Be advised that printers that appear on the unsupported list may never be supported by HPLIP.