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/main/java/org/elasticsearch/TransportVersions.java
if (IDS.add(id) == false) {
            throw new IllegalArgumentException("Version id " + id + " defined twice");
        }
        if (id < IDS.last()) {
            throw new IllegalArgumentException("Version id " + id + " is not defined in the right location. Keep constants sorted");
        }
        return new TransportVersion(id);
    }
    public static final TransportVersion ZERO = def(0);
    public static final TransportVersion V_7_0_0 = def(7_00_00_99);
    public static final TransportVersion V_7_0_1 = def(7_00_01_99);
    public static final TransportVersion V_7_1_0 = def(7_01_00_99);
    public static final TransportVersion V_7_2_0 = def(7_02_00_99);
    public static final TransportVersion V_7_2_1 = def(7_02_01_99);
    public static final TransportVersion V_7_3_0 = def(7_03_00_99);
    public static final TransportVersion V_7_3_2 = def(7_03_02_99);
    public static final TransportVersion V_7_4_0 = def(7_04_00_99);
    public static final TransportVersion V_7_5_0 = def(7_05_00_99);
    public static final TransportVersion V_7_6_0 = def(7_06_00_99);
    public static final TransportVersion V_7_7_0 = def(7_07_00_99);
    public static final TransportVersion V_7_8_0 = def(7_08_00_99);
    public static final TransportVersion V_7_8_1 = def(7_08_01_99);
    public static final TransportVersion V_7_9_0 = def(7_09_00_99);
    public static final TransportVersion V_7_10_0 = def(7_10_00_99);
    public static final TransportVersion V_7_10_1 = def(7_10_01_99);
    public static final TransportVersion V_7_11_0 = def(7_11_00_99);
    public static final TransportVersion V_7_12_0 = def(7_12_00_99);
    public static final TransportVersion V_7_13_0 = def(7_13_00_99);
    public static final TransportVersion V_7_14_0 = def(7_14_00_99);
    public static final TransportVersion V_7_15_0 = def(7_15_00_99);