When will be able to leave any group message. Simply hiding the conversation is not enough. I want to be removed from it.

That's a great question. But Apple doesn't have any control over cellular provided features, which is what you're asking about. You should ask your cellular provider that as many people would agree that not being able to leave a group text can be annoying.

Apple has no control over SMS message behavior. That’s is a function of your wireless carrier. You can only remove yourself from an iMessage conversation, an app over which Apple has control.