I've added Slack to the notification area of the taskbar by going to Settings and finding
Select which icons appear on the taskbar section. I switched Slack on on and all was good.
Eventually, after a month or two, Slack disappears. I go back into
Select which icons appear on the taskbar section and sure enough, Slack is set to off.
I suspect that's because Slack keeps updating itself and the EXE itself is changing. But that's just a guess.
What can I do to fix this problem?
P.S. Clarification. Windows 10 doesn't have "Only Show Notifications" - that's Windows 8.x. It's either on or off. As in image below: