If it’s urgent enough that the actual message isn’t enough, “Hello” isn’t going to cut it either.
I've also had cases where I've immediately responded "hi" only to get the question about 1h later.
In the case where you want an immediate discussion, don't trust the status indicator, and there are multiple different people you could contact, a quick confirmation that the selected party is online and available is not unreasonable.
By the way, I also hate the "hello"-only message. I am, however, guilty of writing "Hey. Do you have a second to chat" - typically in cases where either through chat or video conference I want to go through something that is more involved, and I also want some confirmation of understanding and acknowledgement.
If you want my attention give me a reason to give it.