This is part of the NServiceBus Upgrade Guide from Version 9 to 10, which also includes the following individual upgrade guides for specific components:
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SetClientCertificate supports fewer certificate formats
Due to Microsoft making the X509Certificate2 constructor that accepts content as a string file path obsolete, the SetClientCertificate overload that takes a path and a password now only supports loading X.509 or PKCS12 certificates.
If a different certificate format is required, the SetClientCertificate overload that directly takes an X509Certificate2 instance can be used instead.