A stop sign is the definition of flat: https://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/ST... but it has a border which separates it from the rest of the world. If it were a button when you've clicked it once you will know where it is and how big it is.
Random text with no differentiation from the rest of the page is a big reason why I've lost the habit of clicking hyperlinks. If it doesn't look like it will do something I will assume it does nothing.
As for why zoomers are now into the 90s, I think there is some picked up nostalgia from millennials around the 90s by osmosis, kind of like how there was some nostalgia for the 70s in the early 00s, limited as it was for millennials, which I think was about the time the boomers first had their midlife crisis (fits somewhat for now I think).
I think it's just because that's how long it takes between the invention of a trend or style, and the majority of people not remembering a time when that was 'the new awful thing that the youth of today are doing'.
I'm just glad my college clothes will be back in style again, right as my daughter gets old enough to care what her dad is wearing.
One huge problem with responsive design, especially mobile-first design, is that it largely killed the creativity of the old desktop web. Nobody is going to spend time developing 2 unique designs for 2 form factors. Everybody will go with the easiest option of RWD which is to stack elements on top of each other in the most boring layout possible.