If you are in the habit of accessing Slack on public computers or suspect someone accessed your account in an unauthorized manner, it’s best to log out of Slack on all the devices you might have used. There is no need to manually do this. You can access your account on one device and log out of all the devices at the same time.
1) In your slack web app, click the down arrow next to your profile.
2) In the menu that opens up, select Profile and account.
3) In the right pane, click the menu icon under the profile photo.
4) Select Account settings.
5) In the Account window, scroll down to the end of the page.
6) Click Sign out all other sessions.
7) Enter the password to authorize sign out.
8) Click Sign out all other sessions.