Technically you are correct but this is just pedantry. Of the 4 examples you listed, 3 are corner cases, and 1 is also a variant traditional character. You know way too much not to realize this (by the way, that's a compliment!), and that you couldn't come up with anything reasonably common like the 發/髮 → 发 pair (because there isn't anything comparable going the other way) indirectly proves the point I was making.
Unless at a very advanced level, a person switching from traditional to simplified can simply afford to ignore any of these relatively rarely-encountered one-to-many mappings, whereas the same cannot be said about switching from simplified to traditional. It is then one extra thing to study if going one direction but not the other. That might be good to know for somebody who is actually making that decision, although in reality it's probably moot, since people will learn either one or the other first depending on their individual circumstances.
> I'd be pretty shocked if any literate Chinese speaker in the PRC did not recognize that character.
That might very well be true but is beside the point.
The claim in the parent post was that somebody who only ever learned simplified characters would be able to read traditional characters "on the spot" having never seen them before, to which I provided this counterexample of a commonly-encountered traditional character that will not be recognized without context, unless the person is already familiar with it.
All I'm saying is nobody is likely to figure out this character seeing it for the first time without context. (For the record, this applies both ways, just the example would have to be different.)
> I'm fairly certain the overwhelming majority are due to automated conversions.
But in that case it also says something that the software didn't handle the conversion well (implying that perhaps whoever wrote it didn't know about it), and that people posting this stuff also didn't correct the obvious mistake (suggesting they didn't know either).
*
You wrote a lot in response, and I generally don't disagree, I just don't see how most of it relates to my post. For the record, I'm not claiming it's difficult or easy going one way or another. In fact I was only pointing out the following 3 things:
1. If you only learned one set (whether simplified or traditional), you don't know the other automatically.
2. If people can read characters in context, they are likely just guessing - nothing wrong about it by the way, that's how people read in any language - but it doesn't mean they would really "know" the same characters in isolation.
3. Some common, unrelated traditional characters were merged to become the same simplified character. This is an extra layer of difficulty if you don't already know which these are (as you would if you learned the traditional characters first). (I concede it's also the case the other way, but in my opinion none of it is common enough to worry about at this level of granularity, so the burden is not comparable.)
I'm getting the impression you're looking for something to disagree with among the things I wrote, and I'm sure you'll be able to find it but I'd rather what I wrote be helpful even at the cost of having to simplify things a bit, and thus not being rigorously correct.
In other words, I'm trying to describe the forest, not the individual trees. My audience are the people who are looking at the forest from a distance: if someone is already in the forest, they either know what's inside or can look around on their own. That the forest insiders won't benefit from my description is to be expected. I hope you can recognize that.