So, it appears that the "missing" workspace branch settings are happily living under `Workspace Settings` -> `Git integration` -> `Branch`. I have no clue whether this was always the case, or I and others like me just didn't drill down deep enough.
Why the option to create a new branch, which is then, to all appearances, locked in as the default working branch for the workspace appears in the top-level `Source control` menu is bizarre. Wouldn't it make more sense to have both options in the same place, and have access limited by role? It might even be nice to have a link to the project on dev.azure.com or github in close proximity, thereby passively spreading knowledge of how devs tend to interact with version control, thereby empowering the user (and keeping us from asking stupid questions, which to all appearnces you'll ignore anyway?)
Thanks to lorcan17 for showing us (okay, probably just me) the buried branch change option. Good on you!