app_metadata.userExpiry:[1512536400000 TO * ] OR _missing_: app_metadata.userExpiry
Does not seem to work anymore. The first part of the search works but not the missing part. Not sure if anything changed on the back-end over there.
You should check this answer to a similar question for additional details on what caused this and why did it happen, but long story short there were updates in the search engine that deprecated the use of _missing_; you can replace it with a negated exists check NOT _exists_:[field].
Thanks very much. Auth0 should update their documentations though! 
Yes, I submitted a request to update those docs around the same time I provided the answer. There was another docs page that got updated when we found out about this, but the one you linked was missed in the initial pass so I did this second request now.