Publicly-trusted client authentication does nothing. It's not a thing that should exist, or is needed.

Feel free to start your own non-profit to issue client certs signed by a public authority.

As LE says, most users of client certs are doing mtls and so self-signed is fine.

> they just decided it wasn't worth the effort anymore

That seems disingenuous. Doesn't being in the client cert business now require a lot of extra effort that it didn't before, due entirely to Google's new rule?