The "JAR Index" feature has been dropped from the JAR file specification. JAR Index was a legacy optimization in early JDK releases to allow downloading of JAR files to be postponed when loading applets or other classes over the network. The feature has been disabled since JDK 18, meaning the `META-INF/INDEX.LIST` entry in a JAR file is ignored at run-time.

The system property `jdk.net.URLClassPath.enableJarIndex`, introduced in JDK 18 to re-enable the feature, has been removed. Setting this property no longer has any effect.

As part of the change, the `jar` tool will now output a warning if the `-i` or `--generate-index` options are used.