Eclipse CheckStyle used to host its p2 repos on Bintray, but that service is not available anymore.
The project has now moved to a new URL, http://checkstyle.org/eclipse-cs-update-site/, but that repo only contains recent versions (starting from v8.41). See the GitHub Issue for more details.
For projects which are stuck on older versions, there is no simple recourse. In our case, Eclipse Sirius is still using a CheckStyle configuration from the very old CheckStyle 8.12, hence this project to make the corresponding p2 repo for that specific version available more easily (until we migrate to a more recent version).