However, note that the OpenSSL API compatibility layer doesn't support TLS 1.3 yet. So, if you're using Nginx or something else that doesn't use the LibreSSL API, you're out of luck for now.
This is simply not true. For a lot software you will get 1.3 automatically, for example mutt and irssi pick it up just fine. The existing APIs will work and LibreSSL will prefer TLS 1.3 over 1.2 if clients/servers support it.
OpenSSL offers some additional APIs for certain TLS 1.3 features, but are hardly mandatory.
OpenSSL offers some additional APIs for certain TLS 1.3 features, but are hardly mandatory.