Altium Designer - Create keepout based on board outline
Go to Design -> Board Shape -> Create Primitives From Board Shape.

Then select what layer you want to create the primitives on and how thick you want the outline to be.

I don't have access to Altium at the moment so this is from memory. I use it all the time, though I use a separate mechanical layer specifically for the board outline (usually Mechanical Layer 6 renamed to "BOARD_OUTLINE").