Not sure why but since recently, the Slack icon in bottom right notification area (Windows 10) isn't visible anymore despite Slack app being checked in "Select which icons appear on the taskbar" section. Consequently I need to keep Slack open at all times in my app bar (which I don't want because it's crowded).
Any idea what's causing the problem here?
In Windows 10, you have to right-click on the tray (the area you referred to as "notification area") to be able to customize which icons are visible, choose Properties, and then click the Customize button.
From here, click “Select which icons appear on the taskbar”. Enabled items will show permanently on the tray bar.
Here you have a step-by-step guide to solve this issue.
Keep in mind, however, that the you also have to enable the option to "leave the application running in the notification bar" from Slack, as pointed out by Somnath Muluk