Instead of a particular date, it's more likely it was a slow progression. My 12ga 870 Express was purchased in 98ish. No cosmetic issues except when it got rained on and I didn't re-oil it right away it did rust a few times, which I can't blame on Remington. It functions fine, except it hangs up on aluminum hulled shells. I've never fired the steel shells some are talking about.
A couple buddies of mine bought some 870 Express's later around 02-05. One guy had 4 870 Express shotguns between 12 and 20 gauge. I had a 20 ga as well purchased around that time. Again, no cosmetic issues, but we all had extraction problems with both aluminum and brass cased shells with these shotguns, where the sheel would hang up in the chamber and you couldn't pump the gun without considerable force. It was blatantly obvious whenever we shot together that something was very different about my 870 that was 5 or so years older, compared to the other newer ones. I sold my 20 ga that had problems, but my buddy that had four 870's spent a lot of time on the phone with Remington. Had to send one back because he couldn't get the barrel off, turns out the barrel was warped. As far as the extraction he found if he took a fine file to a particular part of the chamber and removed some material it would fix the extraction problems.
It sounds like issues have continued from there, with Remington either doing a worse job with their parkerizing, or using a lower quality steel supplier.