[TLS] RFC 8446: Reserved HandshakeType variants?

[Christopher Patton <[e-mail]>]

Hi TLS,

Appendix B.3 defines the `HandshakeType` for each message in the handshake
protocol. I'm curious about the history of the RESERVED variants in this
list:

```
     enum {
          hello_request_RESERVED(0),
          client_hello(1),
          server_hello(2),
          hello_verify_request_RESERVED(3),
          new_session_ticket(4),
          end_of_early_data(5),
          hello_retry_request_RESERVED(6),
          encrypted_extensions(8),
          certificate(11),
          server_key_exchange_RESERVED(12),
          certificate_request(13),
          server_hello_done_RESERVED(14),
          certificate_verify(15),
          client_key_exchange_RESERVED(16),
          finished(20),
          certificate_url_RESERVED(21),
          certificate_status_RESERVED(22),
          supplemental_data_RESERVED(23),
          key_update(24),
          message_hash(254),
          (255)
      } HandshakeType;
```

Were these variants from previous TLS versions? Are they there to prevent
extensions, or future TLS versions, from reusing these code points?

Thanks,
Chris P.