Google tracks analytics on the quality of its search results, simply by observing which results users click on, how long it takes them, or whether they don't click on anything at all.
The idea that Search is somehow worsening doesn't make any sense.
What has maybe happened is that Search has improved for the population as whole, but gotten worse for certain subsets of users -- but still a net gain overall. Or alternatively, that the web itself has changed -- that Search is doing better than ever, but some users just don't like what the web offers in response. (E.g. nobody writes straight-up recipes without tons of intro anymore -- but that's also so they have tons of space to insert ads, it's not about SEO. It's nothing to do with Google.)