That is toggleable. Cmd+B on Mac. I usually keep it closed, but it's just a shortcut away when I need it.
> 4% (7 chars) of screen space permanently wasted by line numbers
You can disable that in the settings with:
"gutter": { "line_numbers": false }
> 2.7% (5 chars) of screen space taken up by a gutter
You can also disable the other items in the gutter to free up all of that space.
> So 27% of screen space effectively dead 99% of the time.
You can also press shift+esc at any time to toggle a fullscreen pane of whatever you are working on when you need more space without affecting your editor's state. I don't know the name of that action, I actually found that accidentally.
Edit: I forgot to mention, you can actually disable the tab bar now too if you want even more space. You would just need to rely on the tab switcher feature or file search to move around.