looks like this has been resolved by https://github.com/inaturalist/inaturalist/commit/fc62a4ff3d05235da7110b8918a08f87976cee15.
i had been tracking some old photos that should have been deleted from both https://static.inaturalist.org and https://inaturalist-open-data.s3.amazonaws.com, and i can no longer access these. so it looks like the files have actually been deleted or else the server will no longer grant access to these. either way, this addresses the underlying issue.
it looks like there will be a job that runs automatically in the future to delete these files periodically.