AWS's docs are terrible about including info for previous versions, or even what versions exist, or at least it sure seems that way for the AWS RDS API anyways.
This site seems to be the best way to find info about versions of the different AWS APIs:
Sadly, that site's home page doesn't seem to include anyway to filter for a specific service.
Thankfully, it does seem like you can do it 'manually':
- Open AWS API Changes in your browser.
- Perform an in-page search (via your browser) for the relevant service, e.g. "RDS".
- Click on the header for any 'changes entry' for the relevant service. [Here is an example change entry for RDS.]
- On the changes entry details page, there should be 'breadcrumb' links at the top, ex. "API Changes / Services / Amazon Relational Database Service / 2021-09-10".
- Click the breadcrumb link for the service, e.g. "Amazon Relational Database Service".
Example for RDS API changes: