Unfortunately that is correct. Compatibility with iOS 11 required developers to make certain upgrades to older apps including the original Steambirds. We have stopped work on the original Steambirds and will probably not be able to address this issue. The app should still (mostly) work with older versions of iOS if you have a device still running iOS 10 or lower.
Apple unfortunately removed the app from the App Store in early 2019, so at this point the only way I'm aware of to get the game installed is to sync your device with a computer that happened to have a copy of the game.