You will not lose photos when it is shut off. All of your photo stream photos will still be on the device on which the photos were taken.
For instance, if you have an iPhone and you took photos with the phone that were in the Photo Stream album, they will remain on the phone when the service is shut down. If you have an iPad that used Photo Stream and you did not download the photos from the photo steam that were taken on your phone, you will lose those photos on the iPad but they will still be on the phone.
All of the other albums, and settings that are in the photos app will still be there when photo stream is shut down. None of that will change.
If you imported all of your photos to your computer, you are good to go. You will not lose anything.
There isn’t anything that will take the place of what Photo Stream did. Apple is now suggesting that you use iCloud Photos. Read this also read the articles that are linked in top section of this support article.
About iCloud Photos and My Photo Stream - Apple Support
I’m not a fan of iCloud Photos. You need to have enough space in iCloud to use it and it really isn’t storage but a way to keep photos in sync across devices. Many users will most likely need to buy more iCloud Storage in order to use iCloud Photos.
