Surgeon General’s Warning: Long technical ranty post ahead. Reading this post without due care may be hazardous to your mental health.
In 2.35a and earlier, there were 3 buttons, “Sky”, “Premul” and “Key”. Sky would render the background using the sky set up in the world buttons. Premul would make a picture with a pre-multiplied alpha channel. Key would make an image that was perfect for chromakeying - antialiased interiors and jagged edges - exactly what you’re asking for here. All 3 button’s names made sense, and all 3 were useful, except for one minor detail - none of them made a simple alpha image that was good for compositing in other apps - premul only worked if the program expected it, which many didn’t. Key was bad if you didn’t need the 1-bit alpha style, and sky still had the aliasing effect along the edges with the sky colour.
So in 2.36 they added the “normal” alpha render. However, they had the “inspired” idea of replacing “Key” with this new feature. Of course, this means that “Key” is now a misnomer, since the results cannot be used for chromakeying. It also means that there is no real way to make good chroma-keyable sprites any more. It also means that all the help people give saying “use key!” no longer applies. Why they didn’t add it as a fourth button is beyond me.
They didn’t even mention it in the Release Notes - I only narrowed it down to this release by trying it in each one. It works in 2.35a and doesn’t in 2.36. Though, based on people’s reactions in this thread, in which I made a plugin which generated these “true-alpha” images (before I knew Key had been changed), it seems to have been deliberate.
this thread has some good ideas for doing this without using Key.