There's an extension to static-ct-api, currently implemented by Sunlight logs, that provides a feed of just SANs and CNs: https://github.com/FiloSottile/sunlight/blob/main/names-tile...

For example:

  curl https://tuscolo2026h1.skylight.geomys.org/tile/names/000 | gunzip

(It doesn't deduplicate if the same domain name appears in multiple certificates, but it's still a substantial reduction in bandwidth compared to serving the entire (pre)certificate.)

Merklemap offers that: https://www.merklemap.com/documentation/live-tail

These exist for apex domains; the real use-case is subdomains.

Sure, but the subdomains will be duplicated for the same reasons.