I am trying CirrusSearch (es5 branch) with ElasticSearch 5.x and creating mappings fails with an error:
Couldn't update existing mappings. You may need to reindex.
Here is elasticsearch's error message: mapper_parsing_exception: Unknown Similarity type [default] for field [title]
Appears that 'default' is no longer a valid similarity:
'default' has been renamed to 'classic'
Alternatively, if we want whatever really is the default for elasticsearch (BM25?) then maybe it would be used in cases where similarity is not expicitly defined in the mapping? (e.g. it's omitted)
Or maybe we can add some default similarity in $wgCirrusSearchSimilarityProfiles instead of 'default' => ?
Looks like most Wikipedias use BM25 now (wmf_defaults), except some (e.g. Japanese) that still use 'default'