Is this measurable? I have this impression as well, but wonder whether it's purely subjective.

Oh maybe the decline of findability of specific things is caused by the increase of total things on the web.

It varies. I thought about collecting every instance of bad results along with the IP address region and Private Browsing state etc.

at some point they removed the "show users link he clicked before" and that makes the start of Google usefulness. you used to get Wikipedia and stack overflow reliably on top in 2014, then by 2015 it stopped, where by Google choice of by content farm Henning better at gaming the system idk, but the key was removing history from result bias

I've never seen that in my autocomplete. Must be based on your previous search results.