Quarkus 3.7, planned for release in January 2024, will require Java 17 as the minimal JDK version to build and run applications. One of the reasons is the fact that the OpenJDK community ceased actively supporting Java 11 in September 2023. Next to that, Quarkus dependencies are also upgrading their baseline to Java 17. Quarkus 3.0, released in April 2023, already deprecated support for Java 11.
When enabled, Quarkus gathers build time analytics, which currently indicate that less than eight percent of the users run Quarkus builds on Java 11. Other investigations, such as New Relic's 2023 State of the Java Ecosystem, also show an increase in the usage of Java 11 and later versions and a decline in the usage of Java 8. Oracle already stopped Premier Support for Java 11 in September 2023 and Red Hat will stop Full Support for OpenJDK in October 2024. However, other organizations will continue to support Java 11, such as Adoptium until at least October 2027.
The increased Java baseline means that Quarkus (plugin) developers should now support and test their code against Java 17 and they can make use of the new Java 17 features. However at this point in time, Quarkus doesn't use Java 17 features and they didn't really need them to build Quarkus. One of the main reasons to increase the baseline was Quarkus (plugin) dependencies requiring Java 17, which the Quarkus team wanted to support. For example, Hibernate ORM 7, planned for release in 2024, will require Java 17.
Projects unable to upgrade to Java 17 can continue using Quarkus 3.6, although the project recommends to use Quarkus 3.2 as it's the latest Long Term Support (LTS) supporting Java 11. However, Quarkus LTS versions are only supported for one year, so upgrading Java is still important.
Newer versions, such as Java 21 are also supported, but Quarkus opted to keep the required base line at Java 17, as many projects haven't yet upgraded beyond that Java version.
Quarkus follows the strategy of other Java tools and frameworks requiring Java 17, such as Spring Boot 3 which was released in November 2022. This also implied that all the projects, such as JHipster, that use Spring Boot 3 now also require Java 17. Micronaut 4, released in July 2023, and SonarQube server are other examples of Java projects now requiring Java 17 as a baseline.
More details can be found in the official announcement by the Quarkus team.