The keyboard shortcut is spelt out in full right there in the "Recently Closed Tabs" submenu
That keyboard shortcut is displayed two submenus deep. To get to it, one would have to click the Firefox menu and then do two non-obvious things.
The first non-obvious step is to know that submenus can be opened through hovering. In an application using a more or less standard menu bar, clicking the title of a submenu does nothing. Some users traverse submenus by clicking, especially users brought up on touch-controlled mobile devices that don't even have a hover action or people using desktop applications on a touch screen laptop or all-in-one desktop PC. But in Firefox, clicking the title of a submenu of the top-level Firefox menu opens that submenu's primary item.
The second non-obvious step is to know that Ctrl+Shift+T is inside Recently Closed Tabs, and Recently Closed Tabs is inside History. Finding something buried in a second menu is something one does when exploring an application in depth, not when in a hurry to complete a particular task. A lot of users never explore their web browsers in depth because they are effectively in a hurry every time they're using the web: they just want to get back to the web site or web application that they were using. Besides, I don't know if Recently Closed Tabs even shows an option whose shortcut is Ctrl+Shift+T if no tabs have been closed this session.