This is a regression after porting from ODG to SVG. SVG has no property for such an option. 1) Create a shape, activate "Keep aspect ratio" option 2) Save to .kra 3) Close file 4) Open saved file 5) See that the option is now unchecked.
Wolthera suggested to downgrage it to a 'wishlist' bug. WISHGROUP: Small Usability Fixes