Apple Could Have Brought a Big iPhone X Feature to Older iPhone but Didn't

This isn’t exactly true. See Gruber’s note about it [1].

Basically the 7+ isn’t capable of doing the lighting in real time so Apple didn’t implement the feature for post only figuring it would be a bad experience.

1. https://daringfireball.net/linked/2017/11/20/camera-mode-7-p...