I consider both anti-features for _instant_ messaging since it's easy to misuse by hitting enter first and then editing their message for half an hour. Which people do a lot and is quite annoying since it breaks the flow of conversation.

Talking to a person also has none of those features, yet it's still as popular today as it was 300 years ago. And I view IRC just as the text-version of talking in person. So my expectations are that it has more or less the same feature set.

What about a compromise? If someone edits or deletes a message, those actions could be pushed to the client which chooses how to display that information

I think there are some IRCv3 extensions in the works for that, but imho it will make it even more confusing since not everyone will be seeing the same thing (at least for the edits), but I guess we'll see how widespread the adoption of those extensions will be.

And even with only partial support the delete function (if it's not limited to the user itself) gives way too much power to the ops/moderator (power corrupts. Absolute power corrupts absolutely.).