Don't Worry, You Can Still Play Flappy Bird Even Though It's Gone From The App Store
The game's creator, Dong Nguyen,
But that doesn't mean you can't still play it.
First, if you've already installed Flappy Bird on your iPhone or iPad, it's not going to magically disappear now that it's gone from Apple's App Store. You can still play it, but the app will no longer get any new features or bug fixes. If you accidentally delete Flappy Bird, you may still be able to download it again from the "Purchased" section in the iPhone/iPad App Store, but there's no guarantee it'll stay there. (We were able to re-download Flappy Bird today using this method.)
It's a different story with Android users. The official Flappy Bird app may be gone from the Google Play store for Android, but it's been "cloned" by a lot of developers, meaning you can download the same file to your device from another site.
Before you do that though, you need to give Android permission to download apps from outside the official Google Play store. (Fair warning, this could open up your phone to viruses and malware, so only install stuff from trusted sites.) You can do that by going to the settings menu on your device and enabling the option to install apps from external sources. This option is in a different place on different Android phones, so you'll have to dig around a bit. It's usually under the "Apps" section in preferences.
After that, you direct your Android phone to this link to get Flappy Bird.