Force creating a /s3 link for images (for API/indexing purposes)

Assigning to 4.x as we need all features to be added there first.

I can understand the issue and agree that finding a non-breaking URL is indeed a necessity for your situation. I will note that loading the style generation urls does have a larger performance hit than one might initially think. We currently clear the cache of every page that the source image is utilized on in order to prevent caching generation URL's. This will increase the burden to the site, how much of course depends upon your usage and how frequently the generation URL's are being used and how often the files are used elsewhere on the site.

I'm wondering hos common a need this is as it would, on initial glance, require an API addition on the StreamWrapper plugins level since we can't assume that all plugins will use the s3fs_streamwrapper controller.

Assuming it users the s3fs provided controller it would be something like
Obtain the ITOK from the ImageStyle API.
Generate a link using the s3fs_streamwrapper.image_styles.linkgen route and with the query parameter set to the ITOK.
(similar can be done with the 3.x s3fs.image_styles route although the path needs to be appended manually)

With the flexible plugin architecture of 4.x one could create a plugin that will always generate the S3 urls to avoid the need to manually generate them. This could even be extended to use a custom controller to change caching behavior.

Alternatively, perhaps an image style warmer is a better solution? https://www.drupal.org/project/image_styles_generator → is an option that shows up on quick search.