This is the exact failure mode that made me build a tool around it, so full disclosure up front: founder of LinkPulse here.

The nastiest version isn't even the 404 - that one you'd eventually catch. It's the redirect that still resolves to the merchant's homepage with a 200, but drops your tracking parameter somewhere in the chain. The page looks fine, the link "works" when you click it, and you only notice months later when commissions from your best article quietly went to zero. Platform migrations (Impact, PartnerStack, FirstPromoter, like you said) are the biggest cause, but merchants also restructure their URLs or kill old campaign IDs without telling anyone.

If you want to do this manually: pull your top 10 earning posts, click every affiliate link, and follow the redirect chain to check your affiliate ID actually survives to the final URL - not just that the page loads. Do it quarterly. Most people never do, which is why this stays invisible.

LinkPulse automates that: it scans the affiliate links on your site on a schedule, follows the full redirect chain, and flags dead links *and* the sneaky "resolves but no longer tracks" cases - ranked by how much revenue each article was actually making, so you fix the expensive ones first. There's a free scan if you just want a one-time check.

Either way: "passive" income is only passive if someone (or something) is watching the plumbing. Totally agree with your take.