Which is why your test integration environment has to throw 429 and 5xx errors consistently from day one. The error handling paths are hard to retrofit but easy to do before deployment.
The idea behind forcing your clients to handle 429 isn't because they will do the right thing (although you'd think since ethernet won with exponential and random backup with some max timeouts that would be the default) but because it allows you the flexibility server side to start returning 429s when you need to - maybe just based on some idea of server load, maybe as some way of hard rate limits for a certain caller ID, maybe to do maintenance, whatever. It's the semantics "not processed, not fatal, try again in the future" which is hard to shoe horn into a large deployed number of callers after the fact, without angering people.
