Some users are experiencing an issue with the iOS widget where it's either missing or showing all black. When they try to reinstall, it doesn't appear in the list.
Update Flipboard to the latest version and update your iOS to 15 or newer. The widget will reappear if it's still installed and be available again in the list to reinstall.
Can't upgrade to iOS 15+?
There's an older-style widget still available for now. Instead of the + at the top on your Today view, you'll choose Edit and then Customize at the bottom. Flipboard will appear in the list. Hit the red + symbol next to Flipboard, then Done.