OneNote Sync Keyboard Shortcut, F9 Sync all Notebooks, Shift+F9 Sync Current Notebook.

Question:
What is the sync keyboard shortcut in OneNote?

Answer:
Keyboard Shortcut for Desktop OneNote:
F9  sync changes in all shared notebooks. It cannot sync local notebooks.
Shift+F9 sync changes in the current shared notebook.

View Notebooks Sync Status:
Right click on a notebook, click "Notebook sync status" menu item on pop up context menu.  In Shared Notebook Synchronization window, you can read the shared notebooks sync status.