/var/tmp $ javac TestInnerAssignmentCheck.java

/var/tmp $ cat config.xml
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
        "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
  <module name="TreeWalker">
    <module name="InnerAssignment"/>
  </module>
</module>

/var/tmp $ cat TestInnerAssignmentCheck.java
public class TestInnerAssignmentCheck {
    private boolean flag;

    void getOperation(final String operation) {
        switch (operation) {
            case "Y" -> flag = true;  // violation 
            case "N" -> {
                flag = false;
            }
            default -> throw new UnsupportedOperationException();
        }
    }
}

/var/tmp $ RUN_LOCALE="-Duser.language=en -Duser.country=US"
/var/tmp $ java $RUN_LOCALE -jar checkstyle-10.11.0-all.jar -c config.xml TestInnerAssignmentCheck.java
Starting audit...
[ERROR] /Users/hael/Code/Learn/Tools/checkstyle/TestInnerAssignmentCheck.java:6:30:
     Inner assignments should be avoided. [InnerAssignment]
Audit done.
Checkstyle ends with 1 errors.