It was used on startssl.com, at least before they got in cahoots with wosign and subsequently got de-listed as a CA for various shady practices. I haven't checked since.
Also, I think Chrome is in the process of removing support for <keygen> (and client side certificates in general?), so whatever opportunity existed for making this common has now passed.
I remember this on startssl.com, and experience (or at least Firefox's implementation) was horrible.
-I had to refresh certificate once per year, which is very inconvenient, as I would go to startssl.com once per year total
- Half of the time something would go wrong with keygen in firefox and it would not remember my certificate at all. This is when you discover that all the security is simply bypassed when you send email to support.
- And worst of all, it was not very secure at all -- a malware on my computer would totally steal a certificate and keysniff the password for it. Compare with Google Authenticator or SMS auth, where I would be safe as long as my cell phone is not compromized at the same time.
True, the implementations were lacking some finesse. But a proper keychain type cert store handled by the operating system should in theory be able to protect the certificate against usermode malware (perhaps by only coughing up the certificate to an application signed with the same code signing ID as the application that inserted the certificate, and/or after an elevated password prompt)
Also, I think Chrome is in the process of removing support for <keygen> (and client side certificates in general?), so whatever opportunity existed for making this common has now passed.