I believe Java 20 may have some breaking changes for libraries OR uses. I'd like it updated as well to use 20 or better as some of the libraries have security issues that are fixed with higher Java versions. Some of the libraries may not yet be 20 compatible.
Many OSs let you install multiple Java versions and switch between them with a utility or by using custom .bat files that set the JAVAHOME env variable to the proper version before starting the Java app.
One of the biggest reasons is because of Jetty and Butterfly migrations needed. For now, our current minimum compile target for releases is Java 11, until the work is completed on our Jetty 10 to 11 to 12 double migration that @tfmorris is working on with impacts along with Butterfly 2.0
@RolfBly If you are using Windows OS, then you can download and run our Windows release with embedded Java. That way it won't need or disturb your existing Java installation. Also, you can run OpenRefine from the command line with refine.bat, and in doing so it will use the settings you have configured in the refine.ini where you can take a look and uncomment JAVA_HOME or add or modify any more that you need.