Iron Ore in 9

Iron ore is found in deserts, the generation of it has changed, think of the generation as like a big bunch of grapes

sometimes, some can even generate to the surface (mostly cold or copper)

On my server, we have had zero issues with finding good quantities. Heck, at one point it was becoming hard to find rock and not ore. I find that just takes looking in multiple locations. Under 8.0, I had issues finding good, large quantities, yet on the public servers I saw mines where it looked like the walls were made of ore.

Be patient. If you mine in a certain location and cannot find it, look in another location. Also, keep in mind if you're near the edge of a biome and you're mining down on an angle as most do, you can cross into another biome that does not have it. Stick to the sandstone and you should be good.

In 9 ore seems to be deeper. The default config centers the Iron deposits at 22 below the surface in the deserts.

FYI, iron ore now covers the entire desert at depths of around 20-24. Hell it's everywhere with sandstone so in the grasslands sometimes, all the beaches, etc.