gnome-shell-extension-autohidetopbar 20210104-1 (s390x binary) in ubuntu impish

 This gnome-shell extension automatically hides the topbar, except in overview.
 There is also an option to show the topbar when the mouse pointer approaches
 the edge of the screen.
 .
 Please note that you have to manually enable the extension, for example using
 gnome-tweak-tool.