Some `javax.management` classes contained a feature for Serialization compatibility with very old JMX releases (1.0, 1.1, 1.2), which are from before JMX was an integrated part of the JDK (in JDK 5). This was enabled using the `jmx.serial.form` system property.

This compatibility logic offered no value, and only added complexity, so has been removed. This will not affect any JMX features, or any interactions between JMX features in different JDKs.