Original Apple guidelines started with things like "Simplified Jungian Perception" on page 18 https://archive.org/details/apple-hig
Microsoft collected and analyzed hundreds of thousands of data points about their software. See "No Distaste for Paste" https://web.archive.org/web/20080316101025/http://blogs.msdn...
Now?
Modern designers wouldn't understand what a book is if one hit them in the face. And their "research" is all vibes: "Quantified factors" are "32% increase in subculture perception", "a 34% boost in modernity" and "a 30% jump in rebelliousness" https://design.google/library/expressive-material-design-goo...
It looks to me that the research they did only showed them mockups, not actually using this new design. And why are all color choices so bad right now? They just scream, it puts you on edge just looking at it.
I feel like we saw similar changes with the previous shift where new graduates knew GSuite and MS Office was some the software their parents would complain about. It's my shibboleth for identify my generation of computer users.
Used to be when your morning alarm goes off on the bedside table you can just reach over and swipe right... now there are two buttons at the bottom of the screen and you have to look at it and carefully press the correct one.
Also when setting an alarm it used to be set after you selected the day and time. But now they added an extra 'save' button. I am not the only person who thought they set a morning alarm and got a nasty late surprise.
Just changing things for no reason and making them worse.
Otherwise, the UI stays mostly the same, just becoming a bit more bloated ("finger friendly") with every release.
The most annoying thing for me is the waste of screen space from the bubbles around notifications and menu options. Apparently, having stuff floating now gives a "perception of lightness and motion".