There are many reasons not to do this.
1) If we were to add a feature to be able to tell who is ignoring you, that would pose the problem of ignored people hurling abuse targetted at people that can't defend themselves because they won't see it.
2) Malicious people will intentionally ignore people just to annoy them, since now they know that the victim knows that they have been ignored. Expect abuse.
3) This feature would be catering to people that have done something to annoy others and are being punished by being /ignored. This is like rewarding punished people.
4) The amount of additional client->server->client communication is just not worth the effort, especially considering the above 3 reasons.