This is beyond annoying, why did they delete this feature?
My guess is, that Apple removed the list of unnamed faces in High Sierra, because it is totally unusable in a large library, now that we can no longer delete the faces that we do not want. Previously, in Photos 1 and 2, the list of unnamed faces has been nice and easy to use. We could prune it and keep it small by deleting all automatically detected faces that were no faces at all are the faxes of unknown strangers.
After the update to macOS 10.12.5, when Apple removed the ability to delete any automatically detected faces, my library with roughly 50000 photos had a list of more than 15000 unnamed faces, and 99% of them were faces that were of no interest to me. It was nearly impossible to find the few faces that I wanted to name in this horrible list. like seeking a needle in a haystack.
As long as Apple does no longer allow us to remove the faces we do not care about, there is no point in offering a list of unnamed faces, at least not for the whole library.