Hello, yes the good old days. Before the cloud and microsoft abandoned us.
But really, Whs 2011 I may be wrong but, I don't believe many took to it and the ones that did aren't needing the addins, they are working from the desktop.
But, what addin were you looking for or needing? I don't want to state the obvious but "google' may be all that's left if a addin link is dead. Sorry to say, I think even fewer people were writing code for 2011, it was kind of like windows phone, everybody was waiting and seeing if it would fly and it didn't so nobody is writing apps for it. Kind of sad.
The only addins I have used/using are (Smart) free, (PortForward) Free, and (Disk Management) I paid a small price.
Again, what are you looking for? Something may still be out there.
Good Luck
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Fear the Cloud 