This is a follow-up of
LUCENE-8563: Lucene changed its BM25Similarity implementation to no longer multiply all scores by (k1 + 1). Solr was left unchanged by replacing uses of BM25Similarity with LegacyBM25Similarity which returns the same scores as in 7.x.
This Jira makes the default similarity depend on luceneMatchVersion for back-compat if schema either does not define a similarity of defines SchemaSimilarityFactory. If a user has explicitly defined BM25SimilarityFactory then the new will be used and she will need to replace with LegacyBM25SimilarityFactory if she wants to keep old absolute scores (most often not necessary).
This change is also described in RefGuide and CHANGES.