patch a Debian installation package

unpack the deb file in question.

dpkg-deb -x slack-desktop-LATEST-VERSION-PLATFORM.deb unpack

dpkg-deb --control slack-desktop-LATEST-VERSION-PLATFORM.deb unpack/DEBIAN

edit the file named control and replace the unsupported library with a supported version/alternative

libappindicator3-1 -> libayatana-appindicator3-1

repackage the contents of the folder unpack along with the updated control file

dpkg -b unpack slack-desktop-LATEST-VERSION-PLATFORM-PATCHED.deb

install as usual

dpkg -i slack-desktop-LATEST-VERSION-PLATFORM-PATCHED.deb