If you try to permanently exit a Slack workspace, the UI can feel like one of the more difficult escape rooms of the Internet. Here is how you quit a workspace for good in the web client as of January 2021.
The key is to get to your Account Settings. You can access them directly here: https://slack.com/account/settings If you can login successfully (good luck!), select your workspace and proceed to step 3.
Alternatively, if you are already signed into your workspace, click through UI:
Click your user icon in the top right corner and select “View profile”
Click the “…” button and choose “Account settings”
In your account settings, scroll down and “Deactivate your account”. Make sure the correct workspace name appears in the description!
This will only deactivate your account for this particular workspace, because Slack creates a new account for each workspace.
Hope this helps somebody.