Why does my Group Replication plugin fail in MySQL 5.7.24?
As a workaround, I've managed to get the plugin to load by re-obtaining it from the newest MySQL Community Edition. This is likely to break again at some point.

I'm marking this as closed because the reason was incompatibility with the specific version, and there's nothing to be done apart from hope it gets included in the default MySQL edition. Hopefully this is useful for other people.