This same question was marked as a duplicate, so it never got answered here: Sketch: How to get a feathered mask?
This is not a duplicate. I'm asking about Sketch, not Photoshop.
You can get a feathering effect in some cases using alpha masking. However, this doesn't allow you to feather complex (or some simple) shapes in an easy way.
Alpha masking allows you to mask using an object filled with a gradient that includes transparency. The colours of the gradient are ignored and the transparent (or partially transparent) areas of the gradient become the transparent (or partially transparent) areas of the mask.
To use Alpha masking, select the object you've got set as a mask,
Layer > Mask Mode > Alpha Mask.
Then set up your gradient with partial transparency (alpha > 100%).
There's also a good video on the sketch app site: https://www.sketchapp.com/learn/documentation/shapes/masking/#alpha-masks
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:
...which can be achieved by:
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:
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:
...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.