Is there a keyboard shortcut to "unselect" in Windows Explorer?

You can use Ctrl+Arrow to move among selected items. Ctrl+Space will select/deselect. (Space alone will select but not deselect.)

I also found another quick solution: pressing F5 will refresh the folder and deselect single or multiple items.

"To access this button on the Windows 10 ribbon:
Alt+HSN
Of course the Explorer window must have focus during these keystrokes."

"Here is one more possible answer. It is a combination of three keys.
Press the keyboard-right-click key. Press R (properties). Press Esc.
User warning: This should right click the item, go to properties, and exit the properties menu. Make sure this is what is happening, and that R isn't a key that ends up deleting your item or doing anything else.
Other than that, Ctrl + Space is the best answer I have seen here so far."

"I could not find any documentation on Microsoft's websites regarding Ctrl + Space for deselection of items. However, on an unrelated website, winscp.net/eng/docs/ui_explorer_key, there is documentation regarding WinSCP, a free SFTP/FTP/SCP client for Windows. It uses some shortcuts that are also applicable to the Windows OS, such as Ctrl + Space for file selection/deselect. However Ctrl + Space does work, as stated above.

Anyway, the only different answer so far I could offer is:

Press key Delete to delete your file, then press Ctrl + Z to bring it back. Now you have unselected it.

User warning: it may have unintended consequences. Such as if you have configured files for permanent deletion and may not be brought back easily, as is the case in some external hard disk drives. Use this answer with caution."