It's discontinued, but you can still purchase the device (for fairly low price too!). On top of that, based on Apple's track record for software support, you can expect support for at least another year, if not more. Note that the iPhone 5S, a phone released in 2013 on iOS7, received the iOS12 update not too long ago.
A quick check of Amazon brought up a hundred or so resellers. Unfortunately you can no longer get them via Apple refurb program, but an online retailer like Amazon is probably the next best thing if you are careful to vet the seller.
I'm not sure what HN policies are on sharing retailer links, but amazon, ebay, ali-express are all retailers that sell the iPhone SE. The major US cellular providers also still sell the SE, though at a much higher price.
The SE was launched in 2016, after the 6S. It is on par with many newer Android models, has a great camera, supports AR, and still feels faster and smoother than almost all of them due to iOS and better touch response time.
That doesn't change the fact that the hardware is 3-4 years old. The SE has nothing that's newer than the 6S which launched in fall of 2015. It also has some components that are from the 6 era, so fall 2014. Like I said, it's still a capable phone (I've had the SE since release).