OpenSSL 1.1.1 and later supports TLSv1.3. See here:
This version of OpenSSL and its support for TLSv1.3 are included in RHEL 8 and Ubuntu 18.04. See here:
Based on the documentation and the source code, it appears that MaxScale only supports up to TLSv1.2 at the moment:
We should make sure that MaxScale supports TLSv1.3 when it is built with OpenSSL 1.1.1 or later.