The DEB package provided by Slack on their download page is not compatible with Debian 11.
This script aims at letting you install that package safely by rebuilding the original package fixing the dependencies' discrepancies.
This script does not aim at letting you update the package easily: for every update, you have to download the DEB archive and run that script.
The package is available on that page: https://slack.com/downloads/linux
Run the following script passing the package you downloaded:
./install.sh /path/to/slack-desktop-*.deb