How to take a screenshot on Mac while keeping the aspect ratio of the capture area
As far as I know the exact thing you are trying to do is not natively possible in the current version of macOS Mojave. The current modifiers that do exist for the screenshot tool are as follows:


Holding shift  moves only one of the sides of the capture area.
Holding option scales up all sides of the capture area from the center.
Holding space  allows you to move the capture area around.


Alternatively you can use shiftcommand5    which will give you the onscreen capture controls with a few more options for resizing the capture area, however this doesn't include any options for locking the aspect ratio.