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Found this on another forum.

Allow me to suggest creating a text box input your text. Then place the text box over the cell rotating that text box to the angle you desire. You'll want to adjust the cell height as well to fit the text box.

At present there isn't any other way to rotate text.

It was also suggested

Might be an idea to send feedback to Apple via the option under Numbers on the menu bar.

Here is a link to a user guide for Numbers.
 
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