The IESG recently approved the TLS 1.3 draft-28 for proposed standard and we can expect the real RFC for this protocol version to appear soon (within a few months probably).
TLS 1.3 has been in development for quite some time by now, and a lot of TLS libraries already support it to some extent. At varying draft levels.
curl and libcurl has supported an explicit option to select TLS 1.3 since curl 7.52.0 (December 2016) and assuming you build curl to use a TLS library with support, you've been able to use TLS 1.3 with curl since at least then. The support has gradually been expanded to cover more and more libraries since then.
Today, curl and libcurl support speaking TLS 1.3 if you build it to use one of these fine TLS libraries of a recent enough version:
- Secure Transport (on iOS 11 or later, and macOS 10.13 or later)
GnuTLS seems to be well on their way too. TLS 1.3 support exists in the GnuTLS master branch on gitlab.
curl's TLS 1.3-support makes it possible to select TLS 1.3 as preferred minimum version.