Comment by davorak
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.
That is exactly what happens in WhatsApp though. Maybe the message isn't there anymore but it used to say pretty much exactly that.