I disagree. I start text chats with "hello" all the time. It basically relates to channel choice. For some kinds of messages a near-real-time text chat is the right channel but async email-like flow would be entirely the wrong channel. So I start a convo with "hello" to probe if the other party is available for near-real-time text chat, because away-status indicators are frequently unreliable. There are also special…
I must confess though that by default I am very slow to respond to message that starts with hello. I just don't know if they have a 5s question or 2hr problem. So I don't respond until I can commit the upper bound of time. This will result sometimes in slow responses to quick questions - but over years I started to feel the asker should take some responsibility for their own success. A dangling Hello is just insufficient offer.
Perhaps a good compromise is "hey, can you help me troubleshoot this webserver log?" (30min ask) or "hey do you know who's on shift today" (5s ask) or "hey I need to create a new ops manual for payroll processing, when can we Get together" (4hr ask) . It's polite, not assumptive, still inquires on whether they can help in needed timeframe without committing them, but it provides receiver something to help them reply on their own terms.