Little Snitch 4.0 is highly compatible with OS X El Capitan through macOS Catalina.
The network filter (firewall) deactivates every three hours . However, you can re-activate it as many times as you like. Little Snitch 4.0 for Mac Free Download
If you are running an older Mac, version 4 introduced several major improvements that remain useful: Download Little Snitch - Objective Development Little Snitch 4
Little Snitch is commercial software, but it offers a robust free download that functions as a "demo mode": Little Snitch 4.0 for Mac Free Download
The demo mode provides the same protection and functionality as the paid version.
Little Snitch 4 was designed for older versions of macOS. Compatibility is the most important factor when choosing which version to download: