“Support lifespan” is not what you’re referring to here. All Apple products are supported until 5 years after discontinuation. The SE isn’t losing any features, it’s simply not gaining any.
This! We need to stop this way of implying that Apple will be auto-nuking our devices remotely once they're barred from the next big update.
That's just not true in any way.
No, they won't get any of the new features. But Apple will still provide security updates for them for many years to come.
Look at the iPhone 5s from 2013. It got a security update in September 2021, 8 years after launching. And maybe there'll even be one in 2022, we don't know.
No, I'm not saying that these security updates compare to getting the latest iOS version. Obviously not.
But Apple is still providing updates that make it safe to use an "abandoned" device a good number of years beyond the 5 year mark. That's far more support than most other brands provide, if not way more.