>I feel like my leg is being pulled here.

Most of what I said is a simple Google search away. This will be my last comment:

>I find this inconceivable - it sounds absurd,

This is a fairly poor metric for evaluating the veracity of anything.

>because (a) not all income is reported on those forms anyway, even US based

In my experience, most forms of income in the US have been categorized. If you made money, there very likely is some appropriate form you should fill out. How much information the IRS needs about it varies amongst income types. As an example, I sold stuff online for a short while - did not make much money, but the volume was large (probably in the hundreds of items in one year). When I looked into what I needed to report for tax purposes, I was hoping it would be a simple case of 1. Costs of goods 2. Revenue made, and tax on the net. Nope: I found it fairly burdensome. Had I paid a tax person to do it, I would have spent more on the tax professional than my net profit (which really was very low).

>If you are a US adult who has filed tax returns for a few years, I would expect you to have some experience paying taxes on income that wasn't independently reported. Surely you don't fabricate the documents that are normally sent by the income source, when they aren't present.

No one said anything about "fabricating". If the agency doesn't provide you one, you provide that information to the IRS in some way or other. Sometimes it will literally be filling out the same form. In other cases, the IRS will have a schedule for you to fill out. Perhaps the W-2 was a bad example.

I know someone who sold a property he owned outside of the US. The income generated from the sale had to be reported and capital gains tax had to be paid. Now in the US when you do this, some party (I believe the seller's agent) gives you a simple form which you should attach to your tax return. Since this was a foreign transaction, the other party gave no such form. He considered just reporting it as an investment without that 1099. He consulted some tax folks, and they told him that he should do that, but while the IRS may look the other way, legally he is obligated to fill out the form and attach it. The problem was he did not have access to all the figures needed to fill out the form. They advised him to fill out what he could and likely the IRS will accept his explanation.

I personally have had to deal with the IRS - as others have commented, while it always does seem to work out, they can be very annoying. In my case, I was claiming education credits. Some years after I left school, they wanted proof. It took several back and forths to get them off my back, and the experience was quite negative. They continually said "Yes, if you give us such and such information, it will suffice," and when I gave them said information, they kept saying "Not enough - we want more proof". Had to reach out to the university for help, and they were quite surprised - they said they never had to give out as much information as I was asking for students to get those credits, and I was lucky they still had the extra information that I was requesting.

So yes, perhaps usually the IRS will accept the explanation for missing information in forms you can't get because you don't live in the US. But it's not a guarantee.