It's not perfectly clear here in the states for two reasons. Some type 19 planes (made as early as the late 40s and as late as the early 60s) had rounded irons, but most didn't. I'm sure that's clouded even more by some swapping, but the type study pages state that some of the planes did have those new.
Are they as bad as the irons in type 21 planes? I don't know. Type 20 planes are seen as the first "bad" planes by some (others are more particular). Somewhere around 20 or 21, the process was changed and the planes made less accurate in general. However, my day to day smoother is probably a type 21 plane (it's one of the ones that I got with a round iron). I have used other stanley irons and irons of my own make in it and it's a delight. It in no way feels expensive or classic, but it was easy to flatten, the handles feel good, and the large and inexpensive adjuster setup on it is smooth and easy to use. I may have 25 smoothers at this point - it is my choice for daily use, just not with the original iron.
In my view, it's far more practical to assume that the iron is not guaranteed, no matter the era, and that there aren't any stiff rules of thumb that are absolute. For example, record put out a very convincing marketing article about their laminated irons. when GSP opened on ebay, I looted your shores for a while and then resold most of what I got. The laminated irons are no better than the american stanley irons of unlaminated form, as long as those american irons don't appear to be made in some strange color post-oil/water hardening steel era. Assertions have been made here about stanley's laminated irons - "they're harder, longer wearing". I have seen no evidence of that, as again, the hardest I've ever had from stanley was laminated, but quite a few of the laminated irons are slightly softer than all steel irons of a good vintage.
I separate the plane and iron in my purchasing. If a plane has a rounded top iron, I expect it to be $10 cheaper to compensate and won't buy otherwise. I have and can make irons, so swapping isn't a big deal, and I think it's a better plan for someone to expect there may be purchasing more than one of some things in building a kit rather than trying to find perfect.
The later strange-color (water yellow?) rounded top irons in the US that are made in teh 1960s and 1970s, I will take. You can almost roll a burr on some of them. But the plane must either be perfect, or it had better be $10 less than a more ideal earlier type.
What i've seen of english planes, too, is that the marples and i.sorby planes can have very thin and soft irons, but as long as they're not junk (and that's spec) that's a kind of nice thing for dimensioning.
