Konqueror was originally able to be selected as the default file manager by using kcmshell5 componentchooser ("Default Applications"). Now it does not appear in the list of file manager applications.
This appears to be a result of commit 456dec00 of 26 Sep 2016 ("More cleanup of the "profile" concept") removing kfmclient_dir.desktop and no longer installing it. It's not clear whether this was intentional, but this change restores that file and therefore the ability to select Konqueror as a file manager.