According to the NEST matrix compatibility https://github.com/elastic/elasticsearch-net/blob/master/readme.md#compatibility-matrix, i'm having some troubles!, we worked for a long time with NEST 2.5 client & Elasticsearch 5.4 server without any problem, and now with an in-local test (NEST alway 2.5 version with Elasticsearch 7.5) seems works fine (index creation, indexing, searching ...).
I don't follow your question. So NEST 2.5 works with ES 5.4, and now the same client works with ES 7.5. Are you bothered by the fact that it works when it theoretically shouldn't?
To ensure that it will definitely work, continue to work, and work with all supported ES features, you should still upgrade your nest client to a 7.x version, as per the compat matrix you posted.
Apache, Apache Lucene, Apache Hadoop, Hadoop, HDFS and the yellow elephant
logo are trademarks of the
Apache Software Foundation
in the United States and/or other countries.