Based on the chart for the ore distribution for snapshot 21w10a, but playing on 21w11a, I have mined my way down to Y 16 in the overworld and started strip mining for iron. So far I have yet to come across any iron that wasn't air exposed in a cave or a little underground pond. You can see my mining operation attached, and while it doesn't look like much it's ~2 iron pickaxes worth of work where I only found some iron in the air pocket on the right. So far any of my mining building a staircase down to Y -56, this strip mine, and various long tunnels to find large caves, I've found no iron hidden in stone, just all visibly air exposed on cave surfaces and the like.

Edit:: After investigation there obviously is iron that is not exposed to air (see second attachment), but so far in the survival world I am playing on I have barely been able to find any, specifically on the y level that is supposed to have the most (as far as underground is concerned).