What will happen if I don't turn on iCloud Photos when Apple shuts down My Photo Stream?

Learn more about this transition and how to keep your photos up to date across all your devices and safely stored in iCloud by reading this support article —> Information about the My Photo Stream shutdown - Apple Support

The photos in My Photo Stream are already stored on at least one of your

devices, so as long as you have the device with your originals, you

won’t lose any photos as part of this process. If a photo you want isn't

already in your library on a particular iPhone, iPad, or Mac, make sure

that you save it to your library on that device.

Transfer them manually Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

then sync them to your other devices.