These types of wallets generate recovery seed phrases that are widely supported: When you install the wallet, you will be asked to write down a phrase, and you'll be able to type the same phrase into any of 20 other wallet implementations to recover your funds.
Also, correct me if I'm wrong (long time ago I used iOS), but even if Apple removes the application from the App Store doesn't mean it'll be deleted from your phone automatically. At least I remembered that I was still able to use some apps even after they were deleted from the App Store some years ago.
