Most have shutdown their free products in the past few years, I suspect as a result of having to invest money in their decades-old web applications that no longer function correctly in modern browsers.
One remains, but this blog post warns that they practice unsafe private key practices, so I'm inclined to suggest that none do.
https://davidroessli.com/logs/2019/09/free-smime-certificate...
(Mozilla Persona was a great project idea and a shame it never got enough market share. It didn't end up using personal certificates, though IIRC they explored that as an option and what they did use was technically very close.)
The problem is, nothing stops anybody else creating such a CA. Why should I trust either of them? Why should anybody? Unless a critical mass trust the same CAs it's hopeless.
What you need there is a PKI, a Public Key Infrastructure. A PKI has two components, firstly some technical agreements like what sort of certificates are used, how to write email addresses and so on, for which you can mostly rely upon PKIX in this scenario - but then you also need some consensus on the much more difficult and political trust problem.
Mostly what happens is that PKIs are relatively small and private. EMV ("Chip and PIN" cards) has its own private PKI managed by EMVCo, the RIRs have one for route management called RPKI, lots of businesses have their own. Apple and Microsoft operate PKIs for "code signing" of various sorts.
And there's one obvious public PKI, the Web PKI, which is why your browser agrees with mine that this is https://news.ycombinator.com/
But there isn't one for email. So the far trickier problem than spinning up some "LetsEncrypt like service for personal certificates" is building that PKI.
I don't think they ever really took off (or even existed), and there isn't anything in the Baseline Requirements / EV Guidelines as far as I know.
Nowadays they seem to be a bit of a myth, as there is very little evidence of them online except for use in code signing.