Generally in caves starting with y=10 and below, all cave air is replaced by lava source blocks. There are exceptions - mineshafts, dungeons, certain types of caves can generate free of lava below that level, but finding a cave that goes below y=11 with air instead of lava is more of an exception than a rule. As result the "stat" you're asking for is approaching 50% of all cave volume. If you want the exact data, it can be extracted from the resource distribution table. For more approximate, the graph from the Altitude article on the wiki gives a rough estimate.
This is why most seasoned players recommend digging at between y=11 to y=13. The rationales between the exact choice of the height vary and are subject to a never-ending argument and personal preference, but the general consensus is lava is frequent enough and disruptive enough below that level, that any loss in frequency of diamonds is offset by ability to streamline the mining process without incessant disruptions by lava spilling into your corridor, and you needing to dig around it or struggle through it.
