Dear kaattie,
Thank you for posting in the Microsoft community.
I understand that you need to retrieve the strikethrough button. Based on the section you selected, you are using New Outlook, and I will provide an explanation according to this version.
I have conducted tests in New Outlook. It is possible that you accidentally moved the strikethrough button while customizing the quick access toolbar. Please drag the strikethrough button back to the main quick access toolbar in the location shown in the figure below.
If your problem persists, please share with me some screenshots of the quick access toolbar when you are composing an email, as well as screenshots of the options in the customization settings. This will allow me to help you determine the next steps to solve the problem.
Feel free to post back if you need further assistance. I wish you all the best.
Best Regards,
Ulrica.W - MSFT | Microsoft Community Support Specialist