Cloudflare has been serving traffic over TLS1.3 since September 2016, and both Firefox and Chrome now offer support for this protocol. As of May 15th 2018, Cloudflare will be enabling TLS1.3 support as default for all new Enterprise domains added to Cloudflare. Pre-existing Enterprise domains will not be impacted by this change.
For more information about TLS 1.3 and to learn more about the performance and security benefits of it, take a look at https://www.cloudflare.com/learning-resources/tls-1-3/
From May 15th, if you wish to change your TLS1.3 preference from the new default, you can do so via the Cloudflare dashboard under the Crypto app:
Or via the Client API: