Just to add my experience...
Multiple recent .31.x embarks have been almost exclusively on plains, with plenty of sedimentary rock-types, at least at the surface. My embark areas are often currently chosen for copious (if unspecified) shallow and deep metal and often some form of flux or other found in enough quantities to use for more expensive rock blocks. But zero iron. Usually the chosen home civilisation[1] completely lacks iron as well, if the trading situation is accurate, so maybe this indicates a world-scarcity?
This is invariably on the largest (non-island) default world-size setting template, with the only parameter changes that I ever make being increased gaps between cavern layers for more contiguous stone. (Would that reduce ferrous ore veins?)
Tetrahedrite (yes, I call it "Tetrahydrite", in my head, as well) and galena are often there and give me silver (and, the former, copper) and I get steel bars and steel products (meltable, if not directly usable) from the home civ, but I'm used to having a steel-sparse military.
I dedicate myself to fortresses for a lot of time (playing time, micromanaging makes a game-year take ages) so I'm talking about maybe half a dozen examples all with no iron, which of course is not as conclusive or statistically significant as several dozen in which either none or few have had. Chance (and inadvertent self-selection against the more ideal conditions needed) may easily have given me such a non-ferric outcome.
[1] I generally randomly choose home civs, where there's a choice. It could be I've been picking home civs living in a non-iron locale, consistently.