With Arc Home 1.18 release it introduces the ability for the user to personalize a primary website. This should be used to pre-populate the Primary Website in Composer on the Planning tab (since it moved in the Composer 1.49/1.50 release).
Previously when it was on the Circulate tab the Primary website would pre-populate if you had only selected one website to Circulate you story to, but now that they are on different tabs it's an extra step that the producers are having to select via a dropdown.
It seems like this new personalization is made for this exact thing and hopefully part of the roadmap already. I just want to make sure this request is officially documented.