I think it’s great to have a go back to last page button when finished reading topics, especially on desktop, because in my opinion most user won’t use any keyboard shortcut, I don’t use shortcuts even I use discourse everyday.
unlike android or IOS, we don’t have back button or gesture on desktop, if users want to go back to the topic list or last topic they viewed, they will need to use the back button built in the browser or learn to use shortcut.
if we have a back button put at somewhere like timestamp or end of the topic, user probably will have better experience with discourse.