Pop!_OS uses GNOME Shell for launching apps, switching windows, managing workspaces, etc. Ubuntu and previous versions of GNOME Shell supported “status icons” or “AppIndicators” where installed apps could add arbitrary icons to the shell. In GNOME Shell 3.26, this functionality was removed in favor of other APIs.
However, some apps you use (like Dropbox) might not have been updated and thus might depend on being able to draw a status icon. Fortunately it is easy to add legacy status icon support with a GNOME Shell extension.
gnome-shell-extension-appindicator works well for this and is what we recommend users of Pop!_OS install if they need status icons. To install, open the Terminal app (+T) and type:
sudo apt install gnome-shell-extension-appindicator
…and press Enter. If prompted, type your password and press Enter. The extension is now installed, but you need to enable it.
In the Terminal app or from the Enter a command dialog (Alt+F2), type:
…and press Enter. In the Shell Extensions window that comes up, turn on the KStatusNotifierItem/AppIndicator Support switch.
Lastly, you may need to log out and back in for icons to show up. When you do, any app’s status icons will show up at the right-side of the top panel, next to the system menu.