With iCloud keychain sharing enabled both iOS and OS X seem unable to truly “forget” networks I’ve joined. I remove them using the WiFi menu or advanced preferences but they seem to keep returning. With large numbers of networks the U for locating specific networks is quite frustrating.
These unforgotten networks mean I’m constantly having to adjust what networks I join, or I join the wrong ones.
The best workaround I’ve found is to use Keychain Access from OS X to delete the wifi password for unwanted networks. That seems to propagate correctly and the UI includes search.