The fact that the following messages shouldn't be shown publicly is useful information to convey, and I feel like conveying that avoids wasting the first message -- If it explicitly says that what follows needs to be private.

(The recipient needs more than "hi" to know that what follows is private)

This is a serious issue with the massive increase in video calls and screen sharing (and people being oblivious to or not caring about this). Not just chat but also email notifications with previews. This has become something I actively consider before communicating by text instead of calling someone. It would be great if teams, zoom etc could automatically and by default prevent any notifications from popping into a shared screen. And then there’s still the people who leave their mail and chat opened and just cmd-tab through them or leave them on screen while sharing... oh well :)

This is why I use slack in a web browser, and don’t give it permission to display notifications.

I wish there was a way to permanently disable the “you have disabled notifications” prompts though.