> I did not mean to rant

Not at all, I sympathise, we're all like minded people here!

> One example was sending 1.3M update requests a day to manage state of ~60 entities, that have a total of 3 possible relevant state transitions

> but we've also had situations where 429s result in complaints to their management how our API "doesn't work", "is unreliable" and then demanding refunds / threatening legal action etc

That's painful, and at this point, we're beyond technical solutions, this need human solutions. If they can't realise that they're rate limited because they're basically abusing the API, they need to be told in no uncertain terms.

Of course I understand that it's not that simple, as a backend dev, my "customers" are usually other devs so I can be frank, but when dealing with B2B customers we often have to act like they're not in the wrong.