mastodon.social is one of the many independent Mastodon servers you can use to participate in the fediverse.
The original server operated by the Mastodon gGmbH non-profit

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rennertweb

I tried the unpin option for the Notification column and then struggled hard to get it back. After several useless attemps of Logout/Login, looking in the Preferences/Settings, restart the browser and computer, I was lost. After some hours I spotted the Bell-Icon at the position of the empty red box in my screenshot. Dear mastodon developers, this should be improved. Why not putting it into the settings? There is a Notification corner already