Many admins contacted me recently because they received an email regarding the critical update that Mastodon released.
That was a notification from your Mastodon installation informing you that a security update was released.
On Masto.host there is nothing you need to do because the updates are installed for you.
Admins can change the level or disable those notifications in their Mastodon account by going to Preferences->Notifications and changing from 'Notify on critical updates only'.