I just built up a 23" (~58cm) steel frame (1987 Schwinn Super Sport) that came in at 20.3 lbs without top-end components on it. This is the weight without pedals and water bottle cages, which is how manufacturers state their weights. It has Shimano 105 5800 for the drivetrain and brakes, and an alloy wheelset that is about 1700 grams (not the lightest with 32mm depth). The seatpost, handlebars, and stem are 7075 series aluminum instead of 6061. The tires are Michelin Pro 4 Endurance V2 with butyl tubes.

I did get a pretty large weight savings switching to a threadless carbon fork. I probably took 1.5 pounds off the original quill stem/headset/chromoly fork setup. Not everyone is willing to go this route.

I have no doubt that I could upgrade some drivetrain elements and lighten this up to under 20 lbs without a lot of effort, but it's not a priority. And there are steel frames considerably lighter than the one I started out with. So I would say that it's definitely possible to get a steel frame down below 20 lbs, if you go with a carbon threadless fork. A carbon wheelset would also drop hundreds of grams.