It appears that someone in the past had already put in the necessary work for Qt6 support. Maybe upstream have changed their attitude by now?
Based on the answer it doesn't seems so :/
See also https://github.com/shundhammer/qdirstat/issues/242
It seems to be a very slow moving project anyway; picking that PR as a patchset over current version would still be an option over last-riting the package, or upstream coming to their senses.
Another try.