But iPhone 6 can still download whatever version supported by the latest iOS version on the phone, right?

That catalog is diminishing if not almost completely gone. Even if an app exists, unless it is (mostly) local, chances are that it's so old that it cannot talk to the remote server using the correct API. For mainstream apps, if it launches at all, many will immediately tell you to upgrade to a newer version otherwise it won't work.

There's a world of difference between software dependencies going out of date between many releases and a company deliberately disabling older devices from downloading static ebook files instead of maintaining some sort of basic backward compatibility.

> maintaining some sort of basic backward compatibility

Sounds easy for you to type that out on a forum without having to maintain a two decade old stack, which probably has tons of "software dependencies going out of date"