Just wanted to leave a note that it looks like this was removed for custom request handlers as part of migration efforts in our visualization infrastructure, but I don't believe we did this intentionally (in short: it's a bug). There has been a lot of churn in our visualizations which I explain in more detail in Request/Response handlers in visualizations and the corresponding Github issue.
At the moment I believe this would affect any plugins in 7.5-7.7 registering visualizations which are also using a custom requestHandler that needs access to aggs.
I'm working to get this re-added soon... Here's the Github issue if you'd like to follow along: https://github.com/elastic/kibana/issues/67916