I've tried [The "Sidebar" preferences].
[Currently Open Documents] keeps coming back.
It's possible I'm just being a curmudgeon and recalling the old days where TextWrangler had a "Hit It With a Hammer" setting that would forever rid me of this dubious Shiny New Feature. But I've now changed:
Preferences > Sidebar > Automatically show sidebar to √ Never
Preferences > Sidebar > Reshape window when showing or hiding sidebar to _ Unchecked
Preferences > Sidebar > Add documents to sidebar in alphabetical order to _ Unchecked
Preferences > Sidebar > Show icons in sidebar lists to _ Unchecked
And am confused when Ye Olde Currently Open Documents reappears.
If I could identify what sequence of events predicates this, I could possibly be on to something.
Thanks for the suggestions!