I saw Zorba the week of its release at the theatre so it wasn't forgotten. In fact I do not remember any forgotten movies.

Mockingbird and all the above movies that have been mentioned more than once are all treasures,
Many Non precious old movies could in fact have been destroyed never to be seen again. I wonder about Iron Maidens (about Antique English Tractor races)
Is the Day of the Triffids still seen?,

Realistically if you can still see an old celluloid nitrate movie it is not forgotten.
Discoveries are rare but are possible such as the lost segment of Lost Horizon or some old French Films.