I saw a few weeks ago that there's an OpenSnitch package build on SBo.

My already non-standard kernel needed to be recompiled with two additional options (explained in the README.SLACKWARE) to make OpenSnitch run with eBPF, and I needed to run /etc/rc.d/rc.opensnitchd from rc.local, but the install queue with sbopkg works great, though I built the last package by hand, passing eBPF=yes to the SlackBuild. OpenSnitch runs like a top. It's just as annoying as Little Snitch on macOS is when you first run it, but after a week or so it becomes very much quieter.

I'd forgotten how shocking it is to see just how many third-party tracking and advertising sites many web pages want to connect to. You can find on the OpenSnitch github wiki large lists of tracking and advertising hosts to block all at once--a handy supported feature of OpenSnitch. One block-list file there has more than 420,000 hosts to block. I don't recall a similar feature in Little Snitch.

Little Snitch was the last item that I was looking to replace from macOS on my 2012 MacBook Pro and OpenSnitch is very similar. Many thanks to Isaac Yu for the port and the SlackBuild!