The following does not use EXPRESS_IX_SCAN:

db.bar.insertOne({_id: 1, a: 1, b: 2})
db.bar.createIndex({a: 1}, {unique: true})
db.bar.find({a:1}, {b:1})

isEqualityExpressEligibleQuery() disqualifies queries if they contain any projection at all, which seems unnecessary. Simple projections should be permitted, just as they are for _id predicates.