You can not only delete, but also archive a workspace. If you don't use it anymore, but also don't want to delete it, archiving is the best option, because you can always go back and unarchive it. 😉

Once you archive a workspace, it's no longer visible and you cannot access it anymore.
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Go to Settings – Workspaces settings – scroll down and tick the box.
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You can always view archived workspace in the settings, with the label "archived".

And, of course, you will only see the workspaces you have archived in the settings because once you archive them, it disappears from the Workspaces tab.
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If you want to use the workspace again, just go back, untick the box, and save it.