Strikethrough is a formatted text also known as crossed-out text, it is a horizontal line through the middle of text that use to represent wrong-spelled words in digital content.
You can use the Strikethrough in Google Slides option to strikethrough the text or put a line through text that is either removed or deleted from presentation slides.
How to Strikethrough in Google Slides
Open Google Slides on the desktop
Select and highlight the text from the mouse that you want to cross out
Now in the “Menu bar” click on “Format” then navigate to “Text” and click on “Strikethrough” option

The text will be formatted into Strikethrough in slides. See the below screenshot for reference-

You can also make the text bold, italic, underline as your need.
Strikethrough Google Slides shortcut
To save time while making the presentation slides users can also use the shortcut-
For Mac – Select the text and press “⌘ + Shift + X” from keyboard for Strikethrough on Google Slides.
For Windows – Select the text and press “ALT + SHIFT + 5” keyboard shortcut for crossed out text.