Client Auth certificates, threat or menace?

Let's Encrypt issues the vast majority of signed TLS certificates these days.
They rececently said they will end the option to sign Client Authentication
certificates, and only do the more common Server Authentication.

By my understanding, the only place that a mail system uses Client
Authentication certs is that a submission client can present a cert
for SMTP AUTH rather than a username and a password. It's a niche
feature and the normal way to do it is for the mail system to set up
its own private CA and sign the users' certs, so it can just check
that it sees its signature.
encrypt.

This thread at Let's Encrypt claims that this will break sendmail because it
checks for the Client bit when it's sending mail.  That seems wrong but I
figure it wouldn't hurt to ask.

https://community.letsencrypt.org/t/do-not-remove-tls-client-auth-eku/237427

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