How To Make Design Into Mask Sketch App

The original question was a good one, with a concrete example which is displayed on the sketch help page for Masking, and @Alex Magill's answer is helpful, (though the video is no longer available).

Here is a simple example:

doc example

...which can be achieved by:

insert your portrait
create a rectangle of the same size arranged below the portrait
select the rectangle and "Use as mask"
Layer > Mask > Mask Mode > Alpha Mask
change the mask's Fill mode to radial gradient, and change the right-hand gradient stop to 0% opacity.
tweak to taste: you'll probably need to re-centre the gradient fill, and you may need to add a third gradient stop to control the 'width' of the feathering, without changing the size of the entire gradient fill

However, it doesn't seem possible to feather masks in a way that isn't achieved with a simple gradient.

Because the question title is not limited to a circular mask (or the use of gradients), I'm going to outline the limitations of the current tools as I understand them.

I would like to create a feathered mask using a custom vector, which I had hoped could be achieved by:

create a custom vector shape
add a gaussian blur
arrange shape under the bitmap to be masked
"Use as mask"
Layer > Mask > Mask Mode > Alpha Mask

I see that when I "Use as Mask", the gaussian blur is deactivated, and reactivating it has no effect. I've considered whether the same effect could be achieved in another way, eg:

multiple shapes with gradient fills combined with Union
flatten the blurred shape, and use the resulting bitmap as a mask (undesirable)

...but no luck so far.

It seems that Fill is the only property which determines an alpha mask's effect.

I've submitted a feature request to allow any properties of a layer to contribute to an alpha mask's effect.

Here's a minimal reproducible example using Sketch 52.6.