Slack still has .deb files you just need to scroll down and find the small button for it. https://slack.com/intl/en-gb/downloads/linux search for "Download .DEB app". Last I checked they also have an Ubuntu only dependency that has a Debian equivalent but they don't list it so it won't install unless you modify the dependency list. I used the following script to do so, but I haven't updated in a while.

    #!/bin/sh
    if [ "$#" -eq 1 ]; then\
      tmp_dir="$(mktemp -d)"
      dpkg-deb -R "$1" "${tmp_dir}" && sed -i 's/libappindicator3-1/libayatana-appindicator3-1/g' "${tmp_dir}/DEBIAN/control" && dpkg-deb -b "${tmp_dir}" slack-fixed.deb
    else
      echo "Read it"
    fi