Hi all.  I'm not a sophisticated maya user here, but I have been rendering out animations using pre-arnold renderers by simply batch exporting .pngs using maya 2018 and earlier versions.  This gave me image sequences that AE and PR detected automatically and could be implemented with their alpha channels without any extra steps or processing.

Let's be clear, that workflow, which was taught in a lot of tutorials when I learned maya, doesn't work with arnold in maya 2018.  Whether it's a bug or a feature, it doesn't work the way it used to.  I've read a lot of very complicated explanations about this, but here is a simple solution.

It's not enough, at least in my scenes, to render to .tif, but that's definitely part of the solution.  The other component has to do with the default scene light.  In order to make sure the default light dome isn't rendering as a white background, select the default light and go to the aiSkyDomeLight tab and under "visibility" set camera to zero.

Making only this change and rendering to .tif got me the familiar result with hardly any extra work.  hope this helps some other maya students, teachers and hobbyists.