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").

What @DerStrom8 had suggested is correct. +1 to him. I'd like to elaborate a little.

• Go to Design → Board Shape → Create Primitives from Board Shape
• Pick layer and width in the dialog. Click OK.
  Altium will create a track around the PCB. This isn't the keep-out yet.
• Select this new track around the outline of the PCB.
• Go to Tolls → Convert → Convert Selected Primitives to Keepouts.
  Now you have the keep-out around the board outline.

At the time of writing, I'm running Altium 20.1.10 .

added:

Related function in Altium: Board Outline Clearance .
This kind of clearance is faster to set up, because everything is defined in one rule. It creates a clearance with a constant width for all levels. It doesn't create an actual keep out object in the layout.

I used the board outline clearance rule for setting up the minimum clearance. Then I created keepouts on the board outline where I needed larger clearances.