Comment by grishka
I stayed on Mavericks until late 2016, by which time I had to update to then-current system because I got a new job and needed latest Xcode to compile the iOS app. Surprisingly, I didn't hate the flatter design as much as I anticipated. But of course I still miss skeuomorphism.
The worst transition for me was Big Sur (or, more precisely, Mojave -> Monterey when I bought a new MacBook in 2021). The useless margins, the unification of title bars and toolbars that no one asked for, the borderless buttons (I enabled "show borders" in accessibility settings), SF Symbols (no pixel grid alignment whatsoever), and the redesigned alerts, are the worst. From what I've seen of Tahoe, it makes it even worse, both with the touchscreenification and with the wasted screen area.
I agree; I don’t like the iOSification of the UI/UX, and I also don’t like the increased locked-down nature of macOS, such as having to go through a nag screen whenever I use lldb. I still prefer Macs over Windows for work-issued hardware, but for my personal equipment, I never upgraded beyond Mojave. When combined with the lack of user-serviceability on the latest Macs, I switched back to PCs a few years ago after nearly 15 years on Macs. I have a Ryzen 9 desktop and a Framework 13 laptop. I begrudgingly use Windows 10/11; I would’ve moved to desktop Linux if it weren’t for some proprietary software I need that doesn’t run on Linux. I still have my “trash can” Mac Pro on the rare occasion I need a Mac-only app.