Heirloom filtering at the vendor?
 

I see the AddOn Redprints, and my current UI filters Garrison plans by showing me the ones I already know in a red hue.

Is there such an AddOn that also does this at the Heirloom vendor?

Blizzard native UI does it without any addons. Already known heirlooms are dark and desaturated in the list.

Hmm, then I must have a setting wrong. I'll check my filters.

You won't see it, of course, if you have some custom vendor UI or some addon that overwrites visual effects for some reason, but native UI 100% deliberately checks if vendor item is heirloom and then if that heirloom is known at MerchantFrame.lua line 158, if anybody interested.