You’re right in that we didn’t include this change in the release notes. We didn’t think it would have an impact in plugins that work properly with the API and this is also why there was no warning in advance (and also no deprecation period). I’m sorry that it broke your plugin.
We had other plugins affected with other security restrictions that were also introduced in v8.0 and that were fixed in v8.1. I’d say that in the context of SonarQube plugins, the use of classloaders is very uncommon and shouldn’t be necessary.