LUCENE-6766, where we fixed index-time sorting to have first class support in Lucene's ore, and no longer use SlowCompositeReaderWrapper.
This is a dangerous, long living class, that tries to pretend a set of N segments is actually just a single segment. It's a leaky abstraction, has poor performance, and puts undue pressure on the APIs of new Lucene features to try to keep up this illusion.
LUCENE-6766, finally all usage of this class (except for UninvertedReader tests, which should maybe also move out?) has been removed from Lucene, so I think we should move it to Solr. This may also lead to a solution for LUCENE-7086 since e.g. the class could tap into solr's schema to "know" how to handle points fields properly.