If I have an $or where one branch is an empty $in, the branch appears to be removed in the final plan since it will never produce results. However if this branch is a slightly more complicated predicate like "a < 5 && a>6", the branch is kept under the OR, with empty index bounds. The branch could be removed or made EOF.

const coll = db.coll;
coll.drop();

assert.commandWorked(coll.insert([
    {_id: 0, a: 1, b: 1},
    {_id: 1, a: 2, b: 2},
    {_id: 2, a: 3, b: 3},
]));
coll.createIndex({a: 1})

printjson(coll.find({$or: [{a: 1}, {a: {$in: []}}]}).explain());
// branch is removed

printjson(coll.find({$or: [{a: 1}, {a: {$lt: 5, $gt: 6}}]}).explain());
// branch is kept as an ixscan with empty bounds

Marking this as trivial because as far as I can tell it doesn't affect planning efficiency or the performance of the final plan. This came up as an edge case when introducing plan shape counters in query stats.