Krita has 3 toolbars. They are "File", "Edit", "Brushes and Stuff". The "Edit" toolbar is hidden by default. It holds two very frequently used actions: Undo & Redo. Because the toggling of toolbar visibility is hidden away in an obscure place, people often got confused by why the Undo/Redo buttons cannot be shown, even though "they have already been added to a toolbar". I, for one, got confused by this issue before. And today, a moderator of our local community also got confused, and they reported this to me as a "suspected bug". We both ran into this under the Android version, because we really needed those buttons on a touchscreen. Given how experienced we both were, and yet still fell for it, I think it's safe to assume more people would have been confused by this, as well. I suggest we show the "Edit" toolbar by default, and we should move it next to the "File" toolbar. I think that is where it is supposed to be.
That's a regression, it used to be visible by default not that long ago.