SUMMARY At least as far as when I reinstalled openSUSE Tumbleweed some months ago, KDE Plasma Including the 'Up' toolbar action to Dolphin I think would greatly benefit as a default in Plasma 6 (after hearing about other needs for defaults mentioned in https://pointieststick.com/2023/05/11/plasma-6-better-defaults/) Windows 10 and Windows 11 provide it by default as well to the right of the Back and Forward actions, which I think would be a fine placement as well (top is Windows 11, bottom is my own Dolphin setting) https://imgur.com/a/vj4W0VQ
*forgot to finish typing the first sentence. At least as far as when I reinstalled openSUSE Tumbleweed some months ago, KDE Plasma did not come with this default. If it is default now, or if it's a distro-specific change, then I apologize and this ticket can be closed.
I think it is redundant to put an "Up" button on the toolbar because one can already press the parent folder in the location bar to go "Up".
Yea, I thought a bit more about this and read previous discussions on this. The location bar has all functionality needed to go up the file system hierarchy. There is no need to additionally add an "Up" action. If anyone wants one, they can add it for themselves, but as a default for all users it doesn't make sense when the location bar is not only more convenient (bigger click target) for going one level up in the file system hierarchy but also for going up multiple levels up at once. Marking as intentional.