iPhone 7 has 2GB ram, 7+ has 3GB ram.
iPhone SE orig has an A9 chip with 4” screen.
Once A9 support is dropped, Apple will be done with any device that doesn’t support big.LITTLE architecture and I’m thinking Apple may at the same also just drop support for a 4” screen as well.
At that point, Apple could require big.LITTLE architecture support + RetinaHD screen as the line in the sand.
Then a few years after that, I’d bet the A12 would be the next line in the sand, going forward, all devices must have a third party accessible neural engine, and A11 doesn’t really count as it had a crippled neural engine.