This seems to be more focused around predicting when a budget will be exhausted and taking gradual steps to reduce costs as the budget runs out, rather than a global hard cap.

I.e it’s unclear how this works with per-request billing - if I set a budget of $20, then immediately spam $40 worth of S3 GET requests, will 50% of those requests be blocked or will I be charged $40?

I doubt they’ve added a globally consistent billing/charge counter to the S3 read path, so either I get charged $40 or AWS eats the additional $20.

Only about 15 years later than it should have been introduced