With Gradle 8.11, there is a deprecation notice when not using a JVM 17 because Gradle 9 will require 17. But why 17 and not 21?

This decision is made by the team based on the Java 17/21 adoption stats. While Java gets adopted faster these days, it is not that fast to take only a single recent LTS. E.g. see https://newrelic.com/resources/report/2024-state-of-the-java-ecosystem

More curios if Gradle would benefit of the virtual threads

Well, maybe? It is much bigger change as just switching the minimum version. As you can imagine, after bumping from Java 8+, we already have infinite opportunities to optimize the codebase and adopt new features 😄