From #94080 (comment)

[...] mTLS is one of the most common mechanisms for S2S (service to service) calls. In AAD alone, there are >300 billion mTLS calls every day. (and this is just internal traffic). Our public facing flows also have significant mTLS : https://learn.microsoft.com/en-us/entra/identity/authentication/concept-certificate-based-authentication. Perf improvements in mTLS code paths can save millions of dollars in TLS costs.

Mutual authentication should be already fully supported on Windows, but we currently don't support it on Linux (and not at all on MacOS).

This will need some more thinking and testing. Specifically against cases when we unintentionally restore session with wrong certificate. When we only have one identity for any given server it is pretty simple. But if we have multiple services running on different port (SslStream does not have access to EndPoint) or if we want to maintain multiple identities to same service things do get more tricky.

However, cases where only 1 client certificate is provided (whether via ClientCertificates collection, or CertificateContext, or perhaps even the certificate selection callback) may be simple and safe to implement.