Shay Banon first announced that Elastic would move its Apache 2.0-licensed source code in Elasticsearch and Kibana to be dual licensed under Server Side Public License (SSPL) and the Elastic License. "To be clear, our distributions starting with 7.11 will be provided only under the Elastic License, which does not have any copyleft aspects. If you are building Elasticsearch and/or Kibana from source, you may choose between SSPL and the Elastic License to govern your use of the source code."
In another post Banon added some clarification. "SSPL, a copyleft license based on GPL, aims to provide many of the freedoms of open source, though it is not an OSI approved license and is not considered open source."
There is also this article on why the change was made. "So why the change? AWS and Amazon Elasticsearch Service. They have been doing things that we think are just NOT OK since 2015 and it has only gotten worse. If we don’t stand up to them now, as a successful company and leader in the market, who will?"
The FAQ has additional information. "While we have chosen to avoid confusion by not using the term open source to refer to these products, we will continue to use the word “Open” and “Free and Open.” These are simple ways to describe the fact that the product is free to use, the source code is available, and also applies to our open and collaborative engagement model in GitHub. We remain committed to the principles of open source - transparency, collaboration, and community."