In order to improve memory usage per shard, the _id field is not longer indexed by default. Note, no capability is lost except for the ability to use search queries using the _id field. For that, a new ids query / filter was introduced which does not require the _id field to be indexed (and is actually faster). There is a setting to bring it back if needed.
As a side note, the _type mapping can also be set to index : no, with almost all functionality retained (with a bit slower type filtering) and a new type filter, though this does not improve memory consumption by any noticeable measure.