"In the old days Google used to highlight the ads with a background color … over time they have included smaller and smaller notes; this is, of course, a huge debate in the industry. In fact, people have done studies to prove that a large percentage of users don’t know they are clicking on ads.?"
No link to any said studies? Typical.
Here's 2013 one with the old layout of pale yellow background and indicates 40% of people don't understand the AdWords links are Ads (https://econsultancy.com/blog/62249-40-of-consumers-are-unaware-that-google-adwords-are-adverts
But then there's a follow up in 2014 with the current layout of bold yellow 'Ad' badge next to the URL and no background. It shows 36% don't still understand (https://econsultancy.com/blog/64808-36-of-people-still-don-t-realise-that-google-adwords-are-ads/
An improvement or margin of error? Both are possible. But you would think malicious intent to drive up clicks via obscuring information would have a more pronounced effect.
My experience is that the bold 'Ad' badge to the left of every link is much more noticeable than a pale shading that may or may not be noticeable depending on your monitor and settings.
If you want a look at barely noticeable Ads look at Bing ads. They use a grey background so similar to the page background I can barely see it on my work monitor. Not to mention the phrase 'Ads' is to the top right of all the ads, away from where the eye is drawn to. Look at the heatmaps from the study of Google, almost no one would be looking there!