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