1st thumbs up for softness setting in Rect Mask 2D (works with images, text mesh pro, …) but custom shaders are problem atm (like coffee particles shader, and custom shaders)
2nd using Unity 2019.4.16f1 (first time I found out this feature)
3rd searched documentations, unity forums, google search, tried to searched UnityUI.cginc and did not found anything (at the time of writing this)
4th In custom shaders I use:
float clippingAlpha = UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
5th actual question: Please can someone guide me how to get range 0-1 values, instead of 0 or 1 values?
I found this new feature by accident when noticed new serialized fields after update from 2019.4.11 to 2019.4.20.
And it seems there is not much information about it except your post.
If you download a pack of build-in shaders for the corresponding Unity version you may notice that they have changed the way the clipping works: https://gist.github.com/dotsquid/25c171f639b8af78022d1403adeabbcd
Pay attention at lines 103, 114-115.