While I don't care about the "toxic" part of this, I regularly get asked questions via whisper from people who are "offline" who don't realise they are offline and I can't reply to them.

I think if someone whispers you while offline you should be able to respond regardless of their "status"
Either that or the whisper shouldn't work and they should get a "you cannot whisper someone while you are offline" to prompt them to change their status.