While it is safe (correct) for chunk migration and range deletion to use a multikey index to scan through a chunk range, there can be a negative performance consequence to the shard key compatible index being multikey. The shard key fields themselves are guaranteed to never cause the index to be multikey because the shard key value cannot be an array. However, chunk migration and range deletion may need to scan through multiple index keys for the same document when there's a suffix in the index key pattern which causes the index to be multikey.

As a follow on to the work done in SERVER-6491 we should have the dropIndexes command prevent the only remaining shard key compatible index from being a multikey index.

(function() {
"use strict";

const st = new ShardingTest({mongos: 1, config: 1, shards: 1, rs: {nodes: 1}});

assert.commandWorked(st.s.adminCommand({enableSharding: "test"}));
assert.commandWorked(st.s.adminCommand({shardCollection: "test.test", key: {a: 1}}));
assert.commandWorked(st.s.getCollection("test.test").insert({a: 1, b: [1, 2]}));
assert.commandWorked(st.s.getCollection("test.test").createIndex({a: 1, b: 1}));
assert.commandFailedWithCode(st.s.getCollection("test.test").dropIndex({a: 1}),
                             ErrorCodes.CannotDropShardKeyIndex);

st.stop();
})();