GoKawiilWhile Apple usually supports its devices for a long time, nothing can last forever. It's important to know which products the company is no longer selling or supporting, so you can plan when to make the most of an upgrade and avoid getting burned by buying something outdated.
Apple "leaving behind" a particular model can refer to a few different changes. The two most common are removing the device from its online storefront, or not offering the latest major OS upgrade for it. The former usually happens to products that are a year or two old when a new model arrives, so you'll still find them at third-party retailers and Apple's refurbished shop. When the latter occurs, it's no longer a good idea to buy that model, and you should consider upgrading if you're already using it.
As Apple continues to upgrade its line of watches, tablets, smartphones, laptops, and even displays, it's constantly leaving behind older models, forcing users to consider newly supported alternatives.