Digging into more details:

I've setup OpenSearch 3.0 (with updated ExtensionRequest) and HelloWorld extension compiled with OpenSearch 3.0 snapshot from maven.
As expected the following error occurs:

18:23:55.721 [opensearch[hello-world][generic][T#4]] INFO  org.opensearch.sdk.handlers.EnvironmentSettingsResponseHandler - EnvironmentSettingsRequest failed
org.opensearch.transport.RemoteTransportException: [runTask-0][127.0.0.1:9300][internal:discovery/enviornmentsettings]
Caused by: java.lang.IllegalStateException: Message fully read (request) but more data is expected for requestId [6], action [internal:discovery/enviornmentsettings]; resetting
        at org.opensearch.transport.InboundHandler.newRequest(InboundHandler.java:286) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.InboundHandler.handleRequest(InboundHandler.java:242) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.InboundHandler.messageReceived(InboundHandler.java:132) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.InboundHandler.inboundMessage(InboundHandler.java:114) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.TcpTransport.inboundMessage(TcpTransport.java:769) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.InboundPipeline.forwardFragments(InboundPipeline.java:175) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.InboundPipeline.doHandleBytes(InboundPipeline.java:150) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.InboundPipeline.handleBytes(InboundPipeline.java:115) ~[opensearch-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at org.opensearch.transport.netty4.Netty4MessageChannelHandler.channelRead(Netty4MessageChannelHandler.java:94) ~[transport-netty4-client-3.0.0-SNAPSHOT.jar:3.0.0-SNAPSHOT]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.handler.logging.LoggingHandler.channelRead(LoggingHandler.java:280) ~[netty-handler-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) ~[netty-codec-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:689) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:652) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562) ~[netty-transport-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.89.Final.jar:4.1.89.Final]
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.89.Final.jar:4.1.89.Final]
        at java.lang.Thread.run(Thread.java:832) [?:?]

This is because the InputStream used by TransportMessage reads all bytes and then transforms into java objects.
When excess bytes are sent across there are exceptions.

Here are couple of ideas to navigate through this problem:

Naive approach: OpenSearch does this today with its own backward compatibility and check versions on each TransportRequest/Response.
Cons for this approach is every message being transported do handle them manually.
Pros ofcourse is every message format can explicitly handle them.
Relying on OpenSearch StreamInput: OpenSearch StreamInput extends features of java InputStream to support reading specific data types. It also supports reading Optional data.
As an example:

@Nullable
    public BytesReference readOptionalBytesReference() throws IOException {
        int length = readVInt() - 1;
        if (length < 0) {
            return null;
        }
        return readBytesReference(length);
    }

This works only for specific TransportMessages which has to be cleanly handled by each TransportMessage

Extending OpenSearch StreamInput: Extending OpenSearch StreamInput class to handle unexpected bytes (fewer or higher) and let TransportMessage safely work with the bytes sent/received.
Cons for this approach, partial data may be handed to each message and its upto the message handlers/constructors to convert them into Java objects.
Pros enables sending messages across different message formats for the same message.

Build a message which works with forward compatibility #415

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