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 :)
FWIW, OS X has a do not disturb toggle for this very reason. It's slightly hidden, but easily found with Google. That many people don't bother, is just how people are :/
It's not that notifications can't be disabled by clicking a few extra buttons, it's that those extra buttons are outside the usual workflow. Going into presentation mode in Keynote/Powerpoint/Google Slides doesn't also go into the slack client and disable them, nor does it for other programs that pop up notifications (eg skype, windows update/app store updater), and why should it? Keeping slack confined to the web browser sets this boundary neatly, so long as you don't also allow the web page to display notifications (which chrome will do if you let it).
