iRule to Extract and Repsond back with client IP

I assume you understand that "X-Response" will be returned to the client as an HTTP header. If you want the data to be present in the HTTP payload you'd use the "Content" tag:

when HTTP_REQUEST {
    if { [HTTP::uri] equals "/about/health/status" } {
        HTTP::respond 200 Content "TestOK"
    }
}

As far as the data that you send back, that can come from any values presented in the client's request and potentially a wealth of server side information. For example, to send the client's IP back:

when HTTP_REQUEST {
    if { [HTTP::uri] equals "/about/health/status" } {
        HTTP::respond 200 X-Response "TestOK: [IP::client_addr]"
    }
}