This message is shown when viewing the credits of a page (example: https://translatewiki.net/w/i.php?title=Main_Page&action=credits). Note that this action is disabled by default (currently enabled on translatewiki.net).
This message appears at the very end of the list of names in the message("Based on work by $1.").
If there are no anonymous users in the credits list then this message does not appear at all.
- $1 - the number of anonymous users in the message