The change actually helps this, not the opposite.
The way it used to work:
Whenever you undo a shop purchase, you lose 100% of the health and mana you regenerated between the purchase and the undo. In an extreme case, if you recalled at 1hp, bought an item immediately, waited until you were fully healed, then realized you wanted something else and pressed undo, you would go back to 1hp.
The way it now works:
When you undo a shop purchase, you lose 100% of the health and mana you regenerated between the purchase and the undo, but simultaneously GAIN 100% of the health and mana you WOULD HAVE regenerated without that item. Meaning that you'll have the exact same (unchanged) amount of health and mana if the item you bought didn't give any regeneration stats. If it did give regen stats, you'd lose a very small amount of health and mana that would probably be barely noticeable.
So in the former case, it's very possible to die because of an undo. In the new scenario, it's extremely unlikely.
Although in both cases, purchasing a giant's belt right as you're about to die in the fountain can still save you (although I think taking damage will prevent you from undoing that purchase, so you'd be stuck with the giants belt, even if the global would not have killed you).