We’ve been making a tigged Zona race bike for road and cross for about 2 years, and have refined things just a bit, but the weight hasnt gotten below 1900g for a medium/54cm. These have sliding dropouts, disc brakes, etc. We haven’t made them yet, but have a few life tubesets and fixed dropouts specifically to see what kind of weight we can save in steel, and optimistically it’ll still be 1750g. That’s with a 35mm DT, 25.4 ovalized TT, and ext butted 28.6 ST. IMO and with feedback from the cross racers using our bikes, they’re plenty stiff but could drop some weight. The few folks racing road on them have asked for slightly MORE stiffness.
Rim Brake and component selection is key. There’s a steel rock lobster cx bike in the DC area thats around 16.5 lbs.
There’s pretty different stiffness requirements for road and cross that make it hard to have a bike that’s great for both. Lateral flex in the front triangle is important for good tracking on offcamber and ruts, but not as good for hard cornering on tarmac or, as bobke would say, going “full cry” in a road sprint. I bet @englishcycles has a better understanding than most of tube selection for adequate stiffness and minimum weight.