That’d be useless though, because first, it’d probably opt-in via configuration settings and even if it wasn’t, people would just fork and modify the code base or simply switch to another ActivityPub implementation.
No it wouldn't, because it'd still be significantly easier for instances to deal with CSAM content with this functionality built into the platforms. And I highly doubt there's going to be a mass migration from any Fediverse platform that implements such a feature (though honestly I'd be down to defederate with any instance that takes serious issue with this).