I did several multi day trips to deep wrecks where we could only bring 2 sets of doubles and 2 deco tanks. We had a compressor and 2 k's of O2 on the boat, so we started on the deepest wreck and kept blowing our doubles back with air. I started those trips with 8/92 heliox. The deepest wreck was around 280, so didn't need that mix for the depth, but it let me have decent mixes on the subsequent wrecks.Yeah, I don't think I've ever heard of a mix with an FO2 lower than 10%. If there is, it's not being taught in any curriculum by TDI or the Cave diving agencies. It would have to be something done privately by the dive teams.
When commercial guys were doing sat dives to 800+ feet, they used mixes with even less O2.