Currently, TLS 1.3 is NOT enabled by default on the client-side, because the handshake changed a bit in TLS 1.3, and is considered completed on the client-side as soon as the requested client certificate is sent, i.e. before server validates the presented client certificate. This means that the client-side transport socket reports connection as established, and the client starts sending data without knowing if the server is going to accept the client certificate, which makes handling failures a bit tricky with respect to retries and buffering, and makes enabling TLS 1.3 on client-side significantly more difficult than simply changing the maximum supported protocol version.

cc @mattklein123 @lizan @derekargueta