It wouldn’t surprise me if it is, although we’re just speculating at this point.
The same question has been asked of even the iPhone 6/6+; the 5S will no longer receive patches post-iOS 11, and since it has only 1GB of RAM, there is speculation that the 6 series will share its fate (having 1GB itself).
Now that every flagship Apple device seems to have at least 2GB of RAM, it wouldn’t be wild if that became the lowest common denominator moving forward. We shall see!