With version 7.0 and up Liferay uses Elasticsearch. While Liferay 7.0 defaults to Elasticsearch 2.4, this version is already EoL. It is recommended to install the ES 6 adapter from Marketplace (7.1+ already includes it) and to use an ES 6 version.
Elasticsearch uses Lucene internally, but I couldn't find a list showing which version is used internally. I believe, ES 6.x always(?) uses some Lucene 7 version internally. ES 7 uses Lucene 8.
