eventlogging-valid-mixed was originally created to contain (almost) all eventlogging topics in one topic for backwards compatibility with old Zero-MQ based clients. We then also used it for the eventlogging-consumer MySQL to ingest an includelist of event topics into the eventlogging MySQL database, which was deprecated and removed in T159170: Sunset MySQL data store for eventlogging.
I don't think there are any active consumers of eventlogging-valid-mixed, so we should stop producing to it.