CASSANDRA-10091 we changed the way the JMX server is constructed such that this is always done programatically, which gives us control over the authentication and authorization mechanisms. Previously, when LOCAL_JMX=no, Cassandra would allow the JMX setup to be done by the built in JVM agent, which delegates to sun.management.jmxremote.ConnectorBootstrap to do the actual JMX & RMI setup.
This change has introduced a regression when SSL is enabled for JMX connections, namely that now it is not possible to start C* with only the server-side elements of the SSL setup specified. That is, if enabling SSL with com.sun.management.jmxremote.ssl=true, it should only be necessary to specify a keystore (via javax.net.ssl.keyStore), and a truststore should only be necessary if client authentication is also enabled (com.sun.management.jmxremote.ssl.need.client.auth=true).
As it is, C* cannot currently startup without a truststore containing the server's own certificate, which is clearly a bug.