From time to time, Apple makes the decision to discontinue the supply of spare parts for one of its older products. This makes sense from a logistical point of view, as the stock dwindles and the user base shrinks. But the announcement also serves as a subtle reminder that it's time to buy something new.
The axe is falling again. As of 31 December, the iPhone 6 Plus will be placed on Apple's Vintage List, as it has been more than five years since the handset stopped being sold. The iPhone 6 Plus's sibling, the iPhone 6, continued to be sold until 2018, so that device won't see support discontinued until 2023.
When the gap since a product was sold reaches seven years, it moves from the Vintage List to the Obsolete List, which currently stretches from the original iPhone to the iPhone 4s. Obsolete products lose access to all hardware servicing (except for some MacBook battery repairs); Vintage products can still get some servicing, but only if the parts are available. You can read more about this process in How long does Apple support iPhones?
A full list of products that no longer have official support is available on Apple's website.