How do I recover OneNote for Mac folders from a Time Machine backup?
You could try to recover your data using Time Machine Backup. Chose the backup that is closest in time to when you lost it. Enter Time Machine and select the backup that's closest to the time you lost the data. In that backup select the folder you want to restore and click on the restore button.

If you still can't find this folder or the files you can use a data recovery app to recover deleted files. There are several apps available for this purpose but none are cheap. I successfully used Stellar Date Recovery to recover some files I accidentally deleted. This application has a free trial period which you can use for this. There are others as well. I'm not associated in any way with Stellar Recovery software. It's just the one I tried and it worked.