Cognitive shadows and silver

I was reading up on cognitive shadows during a reread of Mistborn era 1 and came across this:

>szeth-is-my-son-son

>Can every Cognitive Shadow be affected by silver?

>Brandon Sanderson

>RAFO! Good question!

https://wob.coppermind.net/events/360/#e10879

...how and where are they affected by silver at all? Is this a Threnody thing? What's this question referring to?