The Slack Ubuntu/Linux application doesn't provide a way to disable the tray icon and that means the unread or highlight notification bubbles will distract you whenever somebody writes something in ANY channel that you are in, even when the messages are not even interesting for you.
I wrote to Slack support to make a feature request and they suggested to make clever use of an upstream Electron bug that makes the tray icon disappear in 17.04 as a workaround. This gave me the idea for a more stable workaround.
I poked around in the list of files of the `slack-desktop` package and found out that you can just exchange the files for the tray icons , so that the Slack app is fooled into thinking it is showing the unread or highlight icons.
sudo -i
cd /usr/lib/slack/resources/app.asar.unpacked/src/static
mv slack-taskbar-highlight.png slack-taskbar-highlight-backup.png
mv slack-taskbar-unread.png slack-taskbar-unread-backup.png
cp slack-taskbar-rest.png slack-taskbar-highlight.png
cp slack-taskbar-rest.png slack-taskbar-unread.png
Since I probably need to re-run this after every update to the Slack app, I packed this into a shell script `slack-quiet`.