Rather unfortunately, the short answer is no, Fiddler does not support TLS 1.3 yet. The long answer - Fiddler's support for TLS 1.3 is coupled with .NET Framework's support for TLS 1.3. This means that Fiddler can have support for TLS 1.3 only after .NET Framework add support for it. As of this page
there is no word from Microsoft if and when this is going to happen.
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