Because the question has come up from my side following a Github issue regarding ad-hoc groups, I would like to know how Delta Chat should behave when receiving an unencrypted message that suddenly contains a different (reintroducing) key from an already accepted contact to whom we had encrypted communication before. Actually this will happen when this contact e.g. was forced to change the smartphone device, either due to a defect or a loss, and cannot restore the key from a backup because there is none available.
I have been given the hint to use verified groups, so Delta Chat will ignore such messages. Looking at https://github.com/deltachat/deltachat-core-rust/issues/1561 , this behavior seems like a “deadlock” to me: A different key of a (previously) joined group member would result in ignoring such messages at all, without any notice to the recipient. When this happens, the sender has no chance to keep contact anymore.
So when such a message is ignored due to an encryption key change, I believe that some kind of information to the user must be given. Not “bothering” the user with an important information that an (important) message has been ignored is like a little kid who does not want to “bother” the parents while taking to them on the phone by not telling them that actually the kitchen at home is on fire. I do understand that we do not want users to be asked to make decisions, but they should be informed at least that something has been going on.