In Eclipse 2023-06 and onward, external update sites and artifacts must use HTTPS by default even if HTTP is specified: https://eclipse.dev/eclipse/news/4.28/platform.php#force-https

By default, this means a new 2023-06 (and onward) IDE will not be able to resolve our newer Platform Definitions.

The fix is to use the Trust options to allow HTTP for external content, and now the target can be resolved again.