It is in the zope root, see https://github.com/plone/Products.CMFPlone/issues/1927
Ahhhh thanks @alert!
I had the impression that before when you looked at the Undo tab it showed transactions only for the current object, which led me to believe you could find more quickly the transactions you might want to undo. This new location for undo undoes that.
The per object undo stopped working some Plone versions ago... the only reliable way is to undo changes on the db root.
I'm pretty sure I've still been using it in recent versions. But it likely depends on transactions have taken place in the meantime.
I wonder why Zope would make this change to “bury” undo in such an unseen place.