Both the 6s(+) and the iPhone SE (the 5s-shaped one that people love for the small size) will be able to get iOS 14 when it comes out later this year.
Those are the oldest phones which still get new versions of iOS.
My mother has a 6, and it still gets security updates for (IIRC) iOS 11.
It's her second 6, by the way. The battery on the first one started inflating, and they replaced it with a refurbished unit for the price of a new battery.
No other company plays in the same league as Apple when it comes to product support.
I’m planning on keeping my iPhone X past the end of the year, which takes it to a 3 year lifetime, which works out as ~$600/year which is amazing value.