That MySQL notice is standard now in all editions of M2.3.x. It still works, but Magento would prefer that you take advantage of ElasticSearch instead.
However, you can’t simply “turn it on” because you’ll need to install and configure ElasticSearch on your web server first. I started working on a Tutorial for this back in the summer. However, there was a critical change to the Java SDK Licence (Ref).
The Oracle JDK License has changed for releases starting April 16, 2019.
The new Oracle Technology Network License Agreement for Oracle Java SE is substantially different from prior Oracle JDK licenses. The new license permits certain uses, such as personal use and development use, at no cost – but other uses authorized under prior Oracle JDK licenses may no longer be available. Please review the terms carefully before downloading and using this product. An FAQ is available here.
Commercial license and support is available with a low cost Java SE Subscription.
Oracle also provides the latest OpenJDK release under the open source GPL License at jdk.java.net.
This change is super-important because ElasticSearch depends on this technology. Long story-short, the Java SDK License now requires you to purchase a license - Which really complicates the installation process.
I may finish the tutorial at some point, but it’s a bit of a grey area for me creating a tutorial on something that I haven’t purchased a license for.
I appreciate that this doesn’t give you any real resolution, but I hope it helps shed some light on the current situation.