The bottom left corner of notes should have undo and redo buttons. However you won’t see these buttons when you are in multitasking (split screen or slide over) when using a smaller screen. I’m using iPad Pro 9.7” and it doesn’t show these buttons when multitasking.
Hope that helps.
P.S. Also, if you have keyboard shortcuts enabled you can use the undo button top left corner of your onscreen keyboard, but this is a bit fiddly in notes if you want to undo multiple times. If you use this method and it only undoes part of what you want you must press on the screen to get cursor and get rid of highlighted text and then press the undo button again.
Yeah. "Shake to undo" has been available on the iPhone for a long, long time – I think it's waned so much that people might not actually… know about it. I'd presumed it was meant to be the default method of undoing in Notes.