Apple calls a product «obsolete» if 7 years have passed since it was last officially distributed for sale — this means that the Apple Store and Apple Authorized Service Providers will no longer offer repair or other hardware service for the device.
The company has long maintained a list of its obsolete and vintage products (which now includes the iPhone 6 Plus and iPad Mini 4, respectively) — it is publicly available at this link.
Apple released the iPhone 6 and iPhone 6 Plus in September 2014 — the smartphones received key updates such as larger screens and support for Apple Pay. The company discontinued the iPhone 6 Plus in September 2016, but the iPhone 6 remained available through resellers in some countries for several years, so it is not yet considered «obsolete».
As for the software for the iPhone 6 and iPhone 6 Plus, support ended in 2019 for iOS 13.
As for the definition of «vintage» products, which now includes the iPad Mini 4, products are considered to be vintage if they have been distributed for 5 years or more. Apple Stores and Apple Authorized Service Providers continue to offer repairs for such devices for another 2 years, depending on the availability of spare parts.
Apple has also added iPhone 8 and iPhone 8 Plus to its list of «vintage» products, but only in the color (PRODUCT)RED.