When the wheel is locked up (skidding) we can look at the fork and wheel as one rigid body applying a moment to the steerer. The force is applied at the ground and the lever is the length from the ground to the lower headset race. This is the same for rim and disk brakes. The force on the blades is much higher with disk as we know, but the distance from the axle to the disk attach point is so much shorter that we get the same moment up at the steerer.

I'm not advocating for smaller steerers, just saying the brake type doesn't matter. A larger steerer is equally sensible for any type of brake, if stiffer is what you want.

Oh and of course this analysis assumes the rim brake can lock up the front wheel, not always true. But at any given level of braking, steerer flex will be the same. So a stiffer steerer makes sense to whatever degree the disk brake allows stronger braking. If your rim can lock up the front wheel (or eject you over the bars), then the steerer doesn't care which brake. I locked up my front wheel countless times on my old MTB with 1" steerer, but dirt gives lower grip, so the force was not so high. It's difficult but still do-able on pavement, with some fairly extreme body english to not endo. Or on a tandem, which can never endo as far as I know.

Seems to me, tapered steerers for road bikes were only to allow low-density materials (e.g. aluminum) and/or straighter paths for the fibers in CFRP, from steerer to blades at the crown race. I can't imagine a use case for a 1-1/2" steerer in steel, other than triplet tandem maybe. The triplet we made at Santana in the '70s rode and braked OK with a 1" steerer, but I shudder at the thought (no pun intended) — what were they thinking? "OS" steerers definitely existed back then (and back to the 1800s) but Santana was too lazy to tool up for it, until a few years later when 1-1/4" headsets came out.

Mark B