I’m under the impression that as long as the discussion is about using Rack still, it’s fair game? Like, This discussion about the Bespoke Synth wasn’t shut down, and I think that’s because Rack will soon able to be used within it as a plugin, so modular²- though I do see how that could be on the edge as it does fall a bit into competing product territory. A better example would be any of the number of threads about using a VST plugin in host with interesting results.
I too find this rule a bit complicated, as music software and hardware is almost always about how you get multiple things to interact in your setup as a whole. I use my Digitakt and Launchpad Pro as controllers in Rack and interface my hardware rack with an ES-3, ES-6, and FH-2. While I think a discussion about just those things would be off topic, here there’s the obvious context of using them with Rack, and I think there would be some dis-service being done if we were unable to do so, especially as Rack is so weird compared to other DAWs or software that sometimes getting things to play nicely together requires some workarounds or clever patching.