SERVER-127954 conservatively blocks access to fields projected out but disregards the possibility that those fields are renamed (once or multiple times). That is a valid use case that should be permitted. i.e:

view pipeline

{$sort: key1}, {$project: {key1: 0, newKey1: $key1}}

user pipeline:

{$project: {$meta: sortKey}}

will be blocked as key1 is projected out but in reality is visible at newKey1.