It seems like it was a bug, even in the iOS 14 Beta version and it's been reported a lot of times, We hope this issue will be resolved soon. According to the user Heytayjay1 you can download the app call: “Photo Widget:Simple” in the App Store and you can select 10 photos from your camera roll that you want to use as a slideshow. You can press and hold on your home screen to add the widget like normal.