My Photo Stream has been an old syncing service to transfer photos between your devices. It used to upload copies the most recently added photos from the last 30 days into a cache in iCloud, where your other devices could download them from. It did not remove photos from your devices, just upload copies.
If you never used My Photo Stream, there is nothing to worry about. Is My Photo Stream enabled in the Settings > Your Name > iCloud > Photos ? If yes, look for a My Photo Stream album in Photos and check the photos you are seeing there. Save them to an album, if they are not yet in the Imports album.
All Photos you are currently seeing when you open Library view of photos with the Days, Months, Years will remain on your device, when My Photo Stream will be terminated.