I am currently working on a transparent shader and noticed that all my works have a jagged dark outline. I guess I've used the wrong settings somewhere, but I've been fiddling around with a couple settings without any success. I have created a basic shader, which shows the shader settings I use and the problem.
Only your Alpha should use the ellipse output pin.
Your RGB should be one consistent colour, and not darken from the inside of the ellipse to the outside.
For a smoothly-antialiased edge, your AlphaClipThreshold should remain at the default of zero, and you should use Transparent blending rather than Opaque.