Is there a write up of this somewhere? Curious to read more...

None that I found. You can test it right now yourself. Install FB, log in, delete FB, reinstall FB. Your previous login info will be there.

That would be fine if users could SEE what has been stored and DELETE it WITHOUT going through the app and trusting it to show you everything honestly.

What's even worse is that it silently persists across DEVICE reinstalls.

Erase and reset your iPhone/iPad. Sign into the same iCloud account. Reinstall FB. Your login info will still be there.

Buy a new iPhone/iPad. Sign into the same iCloud account. Reinstall FB. Your login info will still be there.

And nope, no one seems to care.