SteamBirds and SteamBirds HD have been removed from the App Store (iOS version)

Unfortunately that is correct. Apple removed SteamBirds HD (the iPad version of the game) on roughly 2019-01-20 because the app "has not been updated in a significant amount of time." SteamBirds (the non-iPad version) was similarly removed by Apple on roughly 2019-03-06. As we no longer work on these games, we do not foresee submitting an update in the near future to get them back onto the App Store.