The Transport Layer Security version 1.3 standard is the latest and greatest version of SSL available. It offers increased security when compared to older TLS standards. By default, cPanel supports multiple TLS versions at the same time for backward compatibility. What are the drawbacks of disabling TLS versions prior to version 1.3?
Answer
An up-to-date listing of browser compatibility can be found here from Can I Use.
Mail client support for TLS v1.3 is limited.
According to Microsoft, and this third-party post, Windows 10 1903 and newer support TLS v1.3; this will affect the most recent versions of Microsoft Outlook running on Windows 10.
Thunderbird also supports TLS v1.3 by default.