Ironstream is the top method I've seen discussed.
https://answers.splunk.com/answers/88172/getting-data-from-mainframe-system.html
https://www.splunk.com/blog/2014/10/01/mainframe-machine-data-in-splunk-made-way-easier/
IBM Common Data Provider for z Systems has also been mentioned in some threads, but with no details to indicate that anyone is using it.