Thanks for your question, Rachelle. We definitely understand the motivation behind removing the sign in link and are working on updated designs to minimize the impact of the sign in bar on the custom headers. That said, the sign in link won't be going away entirely, as having it opens up far more capabilities for the end user, most notably for organizations with non-public content.
By allowing users to sign in, you can grant access to private content presented alongside public content, letting you leverage all of the open data capabilities for any content registered in your ArcGIS Online organization. Additionally for external users, signing in allows them to favorite certain datasets and access their download history, which makes sure they can come back to their research at a later time.
These features might seem wholly unnecessary for your use case, but opening up sites and pages to the ArcGIS named user community is a core requirement for a majority of our user base. We want to make sure things like sign in links aren't too intrusive, so please let us know what you think of our design updates when they roll out in a few months!
