Can older iPhones (with iOS 11) display HEIF images?
There wouldn't be a problem with that. The older iPhone would be able to convert the image back to a readable format.

More info here.

Edit: This will be an issue if trying to view the videos on a mid-2014 MacBook Pro or older. There is ways around it, but only the 2015+ editions can automatically convert them.
I have recently found this out while trying to view videos from my iPhone 8 on my mid-2014 MBP.