I’m not sure this was intentional. Have any of the solutions above worked for you? On my installation, the undo/redo is right where it was in 5.1.5 - I didn’t have to do anything so this has not happened to every user.
They are not really removed¹, but currently you have to set up your toolbar('s) first, to be able to use it. Once you have done it, there is nothing slowing down your workflow.
And like @sooz I don’t know if that was intentional, but I don’t think it was, most probably it was overseen.
But who knows, perhaps it is a measure to make users more familiar with Krita, because now they have to look around where it may be and how to get it into the toolbar they want?