Comment by skywhopper

They did this to stop people from overloading us-east-1a.

It was fine, until there started to be ways of wiring up networks between accounts (eg PrivateLink endpoint services) and you had to figure out which AZ was which so you could be sure you were mapping to the the same AZs in each account.

I built a whole methodology for mapping this out across dozens of AWS accounts, and built lookup tables for our internal infrastructure… and then AWS added the zone ID to AZ metadata so that we could just look it up directly instead.