A family of Microsoft word processing software products for creating web, email, and print documents.
Strikethrough is a font formatting attribute. Select the text then use the button in the Font group on the Home tab of the Ribbon. Strikethrough is right there along with Bold, Italic, & Underline.
If you do much of it you may prefer to use the keyboard shortcut Shift+Command+X or create a Character Style that includes the attribute.