It hasn't gone anywhere though, there are entire countries using it. It was just way ahead of its time and by now has regressed a bit from lack of interested p

It hasn't gone anywhere though, there are entire countries using it. It was just way ahead of its time and by now has regressed a bit from lack of interested parties. TLSv1.3 implementations made renegotiations to mTLS impossible for example, making for example a separate `/login` page not possible. It would be possible to improve the UX, if there'd be an interest. We already have passkeys/webauthn, client certificates aren't much more complex. I also doubt it will go anywhere any time soon for those same reasons.