Personally I expect with the release of iOS 15 we'll probably see this happen. Though Apple is the one with the knowledge of how many iPhone 6 and earlier phones are active in the field (assuming telemetry), so decisions would probably be based off that.

iOS 12 is currently in the n-2 slot, but even on macOS older versions have gotten security updates as needed.

I expect updates it to last one or two more years given their track record. And even when it stops for months, they always come back for emergency security updates.