leaving the country with a late 90 day report (non B visa)

Because I travel a lot, I have done exactly one 90 day report in 15 years of living here. That was recently. So just for fun, I handed my passport to outbound immigration to see if the officer requested it. He didn't. So before leaving the counter, I said "oh, I think I was supposed to give you this [my 90 day report]". I passed it to him, he looked at it and said "what is this?". I just said, well, I think you are supposed to keep it.

Now it may be that failed 90 day reports get flagged in the computer and that pops up when you present to outbound immigration, but who knows. Seems that it was not exactly a high priority with the officer I encountered.

Outbound immigration are not interested in 90 day reports and are not "supposed" to keep 90 day report slips.

OK, I didn't know that. So if outbound immigration are not interested or expected to check for them, how is the requirement enforced, other than when one must visit immigration for an extension of stay?

Immigration at an airport does not even have access to 90 day reporting records those are only maintained at a persons local immigration office where they do the reporting.

The only enforcement is done when applying for extensions or any other contact with the local immigration office.

And even then enforcement is sketchy. I have colleagues who have never completed a report, yet renew their Extensions each year without it being mentioned. It is definitely enforced if you report late.