We pass `null` for the class name here:

https://github.com/elastic/elasticsearch/blob/f086dd18ddc705bf84a1ff99d35196ec2498fd52/server/src/main/java/org/elasticsearch/plugins/PluginDescriptor.java#L261

However the class name is mandatory for the transport protocol today:

https://github.com/elastic/elasticsearch/blob/f086dd18ddc705bf84a1ff99d35196ec2498fd52/server/src/main/java/org/elasticsearch/plugins/PluginDescriptor.java#L156

This prevents various APIs from working in a cluster with stable plugins installed.
server/src/test/java/org/elasticsearch/plugins/PluginsServiceTests.java
final Path plugins = home.resolve("plugins");
        final Path plugin = plugins.resolve("deprecated-plugin");
        Files.createDirectories(plugin);
        PluginTestUtil.writeSimplePluginDescriptor(plugin, "deprecated-plugin", "p.DeprecatedPlugin");
        Path jar = plugin.resolve("impl.jar");
        JarUtils.createJarWithEntries(jar, Map.of("p/DeprecatedPlugin.class", InMemoryJavaCompiler.compile("p.DeprecatedPlugin", """
            package p;
            import org.elasticsearch.cluster.routing.allocation.allocator.ShardsAllocator;
            import org.elasticsearch.common.settings.ClusterSettings;
            import org.elasticsearch.common.settings.Settings;
            import org.elasticsearch.plugins.ClusterPlugin;
            import org.elasticsearch.plugins.Plugin;
            import java.util.Map;
            import java.util.function.Supplier;
            public class DeprecatedPlugin extends Plugin implements ClusterPlugin {
                @Override
                public Map<String, Supplier<ShardsAllocator>> getShardsAllocators(Settings settings, ClusterSettings clusterSettings) {
                    return Map.of();
                }
            }
            """)));
        var pluginService = newPluginsService(settings);
        try {
            assertCriticalWarnings("""
                Plugin class p.DeprecatedPlugin from plugin deprecated-plugin implements deprecated method getShardsAllocators from \
                plugin interface ClusterPlugin. This method will be removed in a future release.""");
        } finally {
            closePluginLoaders(pluginService);
        }
    }