If you want the brush to not create sharp edges on each stroke, depending on the app you can just change the 'softness' setting for the brush. If you are trying to blend and get soft edges you can turn down the opacity and/or flow so the colours don't create solid separations and so the transparency 'mixes' the colours instead of just painting over them. For a lost edge they can be different colours but must be essentially the same value. In the end it's about the transition of values (and colours to an extent) and you can use the methods above to achieve those transitions.

Ha that's the thing, you technically don't need any weird brushes. Soft edges are just some patches of colors/values between two different patches, so you can use any brush to dab some of those colors in between and it will look like a soft edge. Dab more and in a wider range then it will appear to be a lost edge...? Those texture-y brushes are just there so you don't need to manually draw those textures. And blending and smudging brushes as well, it saves you time to not need to draw those patches of "in-between" colors manually.