RFC 4262, "X.509 Certificate Extension for Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities", defines the S/MIME Capabilities Extension which allows to communicate through the X.509 certificate the encryption algorithms supported by an S/MIME recipient whose cryptographic capabilities are still unknown:
Through the extension mechanism defined in this specification, the certificate may also identify the subject’s cryptographic S/MIME capabilities. This may then be used as an optional information resource to select appropriate encryption settings for the communication.
In Windows this extension looks like this:
Now, according to an extensive web search I made, I can see that this extension was supported in old MS Outlook versions but I cannot find any clear reference for any current support for this extension in any email client. I've even looked into the source code for Mozilla Thunderbird as well, but didn't find any usage for this extension.
Do you know of any current software supporting it?