I hope someone will correct me if I'm overlooking something, but I don't think that the OO3 Applescript library gives us access to the selected text within a cell.

(Applying a strikethrough to the entire cell – topic of a row, for example - is less difficult).

You might be able to construct a Keyboard Maestro macro which could navigate its way through the Inspector dialogs (by recognising icons and placing well-aimed and well-timed clicks), and thereby apply a strikethrough to the current selection, but I'm not sure it would be very fast … (bordering on Heath Robinson territory, but possibly diverting …)

Others may spot a smarter route …

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