The pom for undertow-servlet-jakartaee9:2.2.12 declares a dependency on the Java EE 8 Annotations API:

<dependency>
    <groupId>org.jboss.spec.javax.annotation</groupId>
    <artifactId>jboss-annotations-api_1.3_spec</artifactId>
</dependency>

I'm not yet sure if this is just a leftover in the pom, or if the EE 9 class files also have an EE 8 dependency.