It's hard to explain to a non technical user. Something like "We tried to delete the message, but some of the people who received your message might still have a copy." Does not sound great and is going to be hard for a non technical user to understand and hard to implement in a way that a non technical user will find satisfying.

So if I was a dev on matrix/element and this feature came across my plate I would have to weigh it against features that I know can be implemented in a way which make technical and non technical people feel satisfied and better about the application.