It's an issue if you use a CDN or other platform-provided TLS terminator; they don't usually support client certificates. And even if you _can_ support it in y

It's an issue if you use a CDN or other platform-provided TLS terminator; they don't usually support client certificates. And even if you _can_ support it in your reverse proxy and put the certificate details in a header, that's a very different server-side interface from terminating the client TLS connection in the service itself.