The Apple Support Community (ASC) is a user-to-user technical forum. All contributors here are all end-users, just like you.
As for why Apps are withdrawn from the Apple App Store, this will be known only by the App Developer and/or Apple.
Returning to your main question, it may help to understand how modern computer systems, that employ flash-based memory, securely stores data - using iOS/iPadOS as an example…
Your iPad has a fully encrypted file system; all data “at rest” is encrypted.
The encryption keys that are needed to access data on your iPad is stored within the Secure Enclave - this being the security processor of your iPad. The Secure Enclave is logically separated from the rest of the system; it’s function being to securely store all the cryptographic keys that are used by your iPad.
When you enter your passcode, the Secure Enclave is unlocked - and in so doing the crypto’ keys needed to access data on the iPad are released to the Operating system - and are only available within volatile memory while the iPad remains unlocked. As soon as the device is locked, so is the encrypted filesystem and all data stored therein.
If the device/system is erased, is disabled through repeated input of an incorrect Passcode, or receives a remote erase command - all crypto’ keys stored within the Secure Enclave are automatically and securely destroyed. Without the crypto’ keys, none of the data on the iPad can be recovered; a forensic recovery, if attempted, can only recover random data.
This process is known as a crypto-erase. All data is rendered unrecoverable. A crypto-erase is an effective method to render all data permanently beyond reach - and unlike file overwrite techniques (which must overwrite every cell of the flash filesystem), is almost instantaneous.
Solid-state flash storage systems are both over-provisioned and use wear-levelling. Without delving into deep technical discussion as to the implications of how these mechanisms negatively impact the efficacy of file overwrite techniques, perhaps suffice to say that “overwriting” cannot guarantee that all data is effectively erased. Where encrypted filesystems are utilised, fundamental security shortcomings of flash stored can be mitigated.
Modern cryptography with sufficiently strong keys are impossible to crack, within a reasonable timescale, by even the most powerful computer systems that may available to nation-state actors.
If you need to erase an iPad, the best advice is to follow this procedure:
What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support
If you are particularly paranoid, there are commercial software products that can aid with secure erasure. One such specialist in secure data erasure is Blannco:
https://www.blancco.com/
