It appears to be a "back button", e.g. if you click through some sub-menu items on the settings screen, you can see it clearer and it has this functionality.
Re:Keybase 4.0
by tecleandor on Saturday May 11, 2019 @02:33PM
(#19887631)
By its looks, it should hide the side bar...
https://i.imgur.com/nqGfSCL.png