For the Windows version, it states on https://www.digikam.org/download/binary:
We provide also a portable archive file (tar.xz), just decompress the content somewhere convenient, then double-click on the digiKam icon in the folder. This will not impact an installed version of digiKam, though it will share your settings with your regular installed version.
How do I make it fully portable so that I can keep everything on a USB flash drive, including settings, without any changes on the Windows Registry or leave anything behind on the Windows machine?