It can be quite difficult to ‘correct’ that mistake once made, as in order to disable notifications, you often need to open up the chat app itself, which makes all of your conversations flash up on the screen.
Yes, of course you can always just stop sharing, disable notifications, and re-enable, but nevertheless, I always try to actually do the opposite, and send ‘hi’ and then the a follow up message for exactly this reason if I’m sending anything that is confidential or contentious. The difference is that I write the full message first (ctrl + a, ctrl + x, ‘hi’, enter, ctrl + v, enter), so that there’s essentially no delay between ‘hi’ and the content of my message.
You never know.