Hi there, jflynn.
The Include inactive box is located at the gear icon on the Class page. I attached a screenshot for visual reference.

If you don't have the Include inactive box under the Gear icon, I suggest logging in your QuickBooks account using a private browser. This is to verify if we get the same result. And, this method prevents cookies from being stored on your computer, making it a great place to identify browser problems. This might be also the reason the Include inactive box is missing.
Use these keyboard shortcuts based on the browser you're using.
- For Internet Explorer of Firefox, press Ctrl + Shift + P.
- For Chrome, press Ctrl + Shift + N.
- For Safari, hold down Command + Shift + N.
If it works in incognito, I recommend clearing the cache of your regular browser. This removes the history or log of sites so you can start with a clean slate. You can check this article to see the steps: How do I clear my browser cache and temporary Internet files?.
If the steps above didn't work, use another supported browser as alternatives.
Please check this article to see steps on how you can delete or restore a deleted class in QuickBooks Online: Create and manage classes in QuickBooks Online.
Don't hesitate to post again if you have other questions related to QuickBooks. I'm always here to help.