Shake for an undo prompt or three finger tap for a menu that includes undo/redo. Both apply throughout iOS.

Oh, wow, I'm surprised that I keep getting surprised at how undiscoverable ios functionality is. I still can't get over being flabbergasted about a decade ago at how the only way to switch their calculator app to scientific mode is by rotating the phone to landscape mode.

Seriously?

I know that Instagram has this silly feature of "shake to show a 'report a bug' prompt" - I know it because it occasionally turns on for no apparent reason.

It's even less discoverable than any undocumented gesture out there. Percussive maintenance is a joke, and the only people who actually shake something in anger are fictional characters in movies and animation, because that's done for comedic effect. And maybe people with serious anger management issues. Neither connotation make this gesture a sensible choice.

I always thought shake-to-undo was a reference to resetting an Etch A Sketch.

It’s not discoverable, but it is in the user guide.

If you find yourself accidentally activating it, you can turn it off in accessibility settings.

If you would rather have a button for it, you can turn on assistive touch.

Physically shaking the device triggers a confirmation dialog to undo actions in many apps, including in Mail.

All of the other child replies are correct that there are ways around this. But the fact remains that the default set of interacting behaviors (the one I described) is crap.