Since NodeJS 20 became an LTS long after SonarQube v9.9 LTS was released, we don’t expect to go back and declare compatibility. That said, we don’t expect any issues using NodeJS 20. But we haven’t tested it.
Starting from the next version of SonarQube (v10.3) the NodeJS distribution used by analysis will be embedded with SonarQube, so there will be no more need to handle this (or deal with these warnings).
If you use NodeJS 20, you can also point sonar.nodejs.executable to a different version (16 or 18) to avoid this warning (docs)