regardless of if it was in iOS7 or earlier, this should be included.Don't think you ever could.
This is weird, but true. When I search for people who I've had an active conversation with over the last couple days they turn up in the results. However, if the conversation has been dormant since prior to my new phone arriving, it does not show up.You can and always have been able to
However after you have restored your iphone 6 from a previous backup it would seem you have to create a new message to a user before it will search for their name.
did you try searching a name or a word in a message from before your most recent restore (if you have a new phone, try searching a message/person from before you got the phone).I just tried it in iOS 8. I was able to search by first, middle, or last name. Not sure why you are having trouble with this