I've dealt with ES in an air-gapped environment. The problem is that you'll need threat intelligence feeds in later stages of the project to correlate your event with Indicators of Compromise (IOCs). You could use it without, of course, but this would mean you'll be missing quite some valuable information.
Another way could be, set-up an automated way of downlaoding threat intel feeds and putting them into CSV files which then can get ingested locally by the ES SH.
Skalli
