What is an "official API"?

Honest question, because AFAIK there's no guarantee or (legal) requirement to support any API. Whether that's fully documented, has SDKs or whether it's something reversed-engineered doesn't matter WRT the support the company owning the API is supposed or required to give.

Or am I wrong there?

No API changed. They just, almost for sure, decreased their rate limits.

It affects using Aurora Store "anonymously" because that means using shared accounts, so far higher activity per account.

It's possible they're also detecting contemporary usage of the same account.

But there's a slight chance that it's just due to someone abusing the accounts outside Aurora Store.