How to not spell check a presentation in PPT 2019-2024

 Go to the Review tab and select the Language button, then Set Proofing Language.

At the top of the dialog box, you can now select Document - Slide - or Selected Text radio button. 

Then, at the bottom, check the box for Don't check spelling and click OK.

This is how it works in PPT 2019, no idea if it still works the same way in PPT 2021 and PPT 2024. If not please contact me using the reply form on my blog.