Subpages that are removed from the navigation menu can still be accessed using direct links. However, if the subpage is hidden, it can no longer be accessed at all.
To remove the subpage from the navigation menu:
Under the Navigation section, on the left editing panel, hover over the subpage and click the Edit button that shows next to it
Tick the box for Don't show in navigation.
Note: You'll now see the icon below once a subpage is removed from the navigation menu. To put it back, just go through the same process but un-check the box on step number 2.