Gnome's reduction of customizability began in the early millennium when it partnered with some large companies who had carried out formal UI studies and found that for the vast majority of users, options only confuse them.
And it's probably true - give most people a system that is set up for them and they are probably happier than having lots of options. The problem with this, of course, is that "set up for them" is different for each user, and out of the box it isn't really set up right for anyone.
They also made some bonkers design decisions that didn't reduce the configurability but not the complexity of the UI - for example, for a long time they claimed no one needed to turn off DPMS, so the "turn off screen" option just had a list of timeouts (5 minutes, 30 minutes, etc). Sticking a "Never" option in there wouldn't have increased the complexity at all because thats exactly where you would expect to find that option.
And bonkers design decisions that increase the complexity of the UI for no reason - for example, how do you suspend a machine? Oh that's right you press alt while the system menu is open and the power-off button changes to a suspend button. That's a completely non-discoverable design - the only way you're going to figure that out is by reading the manual. How is it better than just sticking a "suspend" button in the menu too, or adding suspend options to the power-off dialogue?
I still haven't figured out how to properly control the screen brightness when on battery power - as of a few versions ago, my laptop screen automagically goes dim when I unplug power. I can turn it up again, but the next time I unplug it it goes dim again. There's nothing in the power or display settings to configure this - the "power" config page doesn't specify separate settings for mains or battery mode; there's just one slider for screen brightness which doesn't seem to control the default brightness for when you're on battery.
And how do you get stuff like you IM client to start automagically when you log in? When Gnome 3 first appeared, it had empathy built into the UI so it was running all the time. These days I have to manually start it up when I log in - can't see any "auto start" button anywhere.
I wouldn't consider things like disabling the DPMS, suspending a laptop, telling the screen not to go unreadably dim every time you unplug the power, or wanting to auto-start background stuff like your IM client on login, to be "power user" tweaks.