this does not make sense to me. ttls tend to be short lived, but as i understand expirys are typically measured in days for this exact reason. if the ttl expire

this does not make sense to me. ttls tend to be short lived, but as i understand expirys are typically measured in days for this exact reason. if the ttl expires, and the upstream cannot be contacted, retry every retry seconds, and in the meantime, if you have the records (every isp has/had records for fb before the outage) that have not expired, then serve them. why did the public user dns servers (isps/google/cloudflare/etc) drop the facebook records? dns is designed to be able to handle an unreachable server. (or was some mitigation for some sort of abuse added along the line somewhere that invalidates zones if they're associated with completely unroutable addresses?)