Is it generally considered acceptable for app owners to be able to retrieve private keys from users when their apps offer end-to-end encryption? E2EE promises that data sending between two endpoints can be decrypted by only those two endpoints. If an app offers E2EE but still has functions to retrieve private keys from users for any purposes such as for multi-device sync, is that considered acceptable as the app owner can get the private keys from the users and decrypt user messages when they need as the app resides on the user devices has the functions for retrieving user private key which could be invoked for other purposes such as for legal execution ?
If you define E2EE as the promise "that data sending between two endpoints can be decrypted by only those two endpoints", then by definition, if the app owner can decrypt, what you have is not E2EE and you shouldn't advertise it as such.