With the release of 3.4 we introduced collations. Among other benefits, that allows MongoDB users to create case-insensitive indexes.

Some of these users (not unreasonably) expect that $regex case insensitive queries should be able take advantage of such indexes. Unfortunately that is not the case - the regex implementation is not collation-aware and is unable to utilize case-insensitive indexes.

We should have this explained somewhere in the documentation about $regex index use.