Hide unused fields in Kibana Discover

I'm seeing the same behaviour using the Filebeat Index template after upgrading from 7.7 to 7.8.

As a workaround you can use Source Filters in the Index Pattern but it would be good to know if there's a way to revert back to the previous behaviour. I also can't see anything obvious about this change in behaviour in the release notes for either ES or Kibana.