Never say never. Judging by the number of duplicates for https://bugs.kde.org/show_bug.cgi?id=290747, I think this is something we should at least be open to doing, no matter how much of a pain in the butt it might be. :)
*** Bug 393634 has been marked as a duplicate of this bug. ***
Now that the port to Q4/Q5 is over and Dolphin is quite stable (for me at least in Debian Testing), maybe we can consider adding back this much requested feature? This feature was removed originally due to low usage but high complexity which complicated maintenance. Maybe maintenance is a smaller burden now that the ports are over and we have successfully replaced Konqueror for file management?
*** Bug 290747 has been marked as a duplicate of this bug. ***
*** This bug has been confirmed by popular vote. ***
*** Bug 434665 has been marked as a duplicate of this bug. ***
Created attachment 141354 [details]
Screenshot of DOlphin's Show in Groups option
As of Dolphin 21.08.1, setting View Mode to Compact and enabling the "Show in Groups" option divides the content of the folder into columns. (See attached screenshot)
Maybe this could be a starting point for a miller-columns-style navigation system?
I have not looked into the source code, I'm just throwing an idea :)