I tend to agree with this, but a friend of mine and I have had a number of conversations that could be potentially embarrassing if anyone else saw them, like during a presentation or meeting. If this is going to be a potentially "eyes only" convo, we'll start with "cyt" for "can you talk". Any answer but an affirmative is taken to be "no", including no answer. Works great.
Even in real life, it’s good practice to say, “hi I have a question. Is this a good time?” It gives the party the option to say “yes” or “give me 5 minutes?”
Just saying “hi” isn’t good online etiquette in my opinion.