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A summary of important changes is published in the Solr Reference Guide at https://lucene.apache.org/solr/guide/8_4/solr-upgrade-notes.html. For the most exhaustive list, see the full release notes here https://lucene.apache.org/solr/8_4_0/changes/Changes.html or by viewing the CHANGES.txt file accompanying the distribution. Solr's release notes usually don't include Lucene layer changes. Lucene's release notes are at https://lucene.apache.org/core/8_4_0/changes/Changes.html