Slack-desktop

wget https://packagecloud.io/install/repositories/slacktechnologies/slack/script.deb.sh
chmod +x script.deb.sh
sudo os=debian dist=jessie ./script.deb.sh
 
sudo apt install slack-desktop

For debian Bullseye, download missing libraries from olde release:

wget http://ftp.us.debian.org/debian/pool/main/liba/libappindicator/libappindicator3-1_0.4.92-7_amd64.deb
wget http://ftp.us.debian.org/debian/pool/main/libi/libindicator/libindicator3-7_0.5.0-4_amd64.deb
sudo dpkg -i libindicator3-7_0.5.0-4_amd64.deb 
sudo dpkg -i libappindicator3-1_0.4.92-7_amd64.deb