It's starting the transition and you can see it on the new system preferences menu
Look at the icons for Battery, Notifications, and others:
Definitely not as flat as Catalina (Or Yosemite). Also look at the battery icon under the Battery status menu, a hallmark right out of iOS 6's charging screen:
The icons in the dock have taken on a far more skeuo look, such as Messages, App Store, System Prefs, Stickies, Chess, Mail, Maps, and Safari to name a few.
It might not be entirely skeuo but it's transitioning there. Assuming the naysayers don't force Apple to stick to flat for another decade plus (please tell me flat ain't forever!) the next OS version could take us even deeper to skeuomorphism, or, as it's known today, 'Neumorphism'. Windows 10 has been doing something similar as of version 1903.
I still love and adore skeuomorphism personally. It was fun to use, made me actually want to touch the touchscreen. My life in 2009 was that of still clinging to a Nokia 5185i on Page Plus, which had the flattest UI ever imagined (it was a literal LCD display, no color, no more than a calculator display.) and at that time I was dead-set against smartphones in general for being overly complicated (as was the case with Palm Treos, Nokia N95s, and the T-Mobile Sidekick) as well as in my view, a solution to a problem that didn't exist. When Apple launched the iPhone in 2007, I regarded it as nothing more than the second coming of the Newton, destined to fail. When batteries for my Nokia no longer held sufficient charge and no one made new ones, I was given an iPhone 3GS (new in 2009) from my boss who tired of my Nokia dying when he needed to reach me. That was my literal first connection to not only smartphones, but iPhone. I ended up adoring that thing. The interface made perfect sense. It was fun to use. It had a radio app that used multi-touch to 'rotate' the volume and tuning 'knob' (some third-party app from long ago) that just made me love it more. More apps kept coming that made it well, to coin a phrase, 'the funnest iPhone ever'. Made me want an iPad (the mentioned iPad 3, which got Siri in iOS 6) and an even newer iPhone (iPhone 4).
When iOS 7 came out, at first I thought my devices were in safe mode as it was stark white and looked nothing like it should. Both my iPhone 4 and iPad 3 self-installed it overnight while they were plugged in (I guess auto updates were on, which, until iOS 7, never really bothered me as they improved and made it even better, well, until iOS 7 came out)
I got rid of them and looked in a cellular store and the only phones doing skeuo at that time in 2013 (well into 2014 too) were the Galaxy S3, and Galaxy S4. I started with an S3, ended up loving Samsung's interface even more, dubbed Touchwiz Nature UX, and being an avid nature/deer lover myself, I really thought it was perfect, from its water drop touch sound to the alarm music, to that iconic 'over the horizon' ringtone. It took longer for Samsung to retire skeuo but they finally did when the S5 came out, and it only got worse from there when they ditched the nature influence completely for One UI. So for now I stick to older Galaxy phones (I got an S5, running a older theme that resembles TouchWiz Nature UX, and a Galaxy S4 I recently got new from Amazon (unopened sealed box) that still has Jelly Bean and with that the skeuo UI that my S3 and older S4 had. I also got an HTC Thunderbolt which works 100%. My only exception is the Samsung Galaxy A01 I'm using as my daily for now, since my girlfriend hated not being able to send emojis to the older phones. It's running a theme (thanks to Samsung Theme Store) that brings back the skeuo icons from older TouchWiz, and another theme on top of that one that restores a lot of the skeuo UI so it looks more like a 'what would TouchWix Nature UX look like if it were being developed in 2020' UI. While I cannot restore the touch sounds or alarm music, I was able to sideload the ringtone from the early times. Not a bad phone but I keep my others just in case I end up hating it later on.
So I am aware of flat design and I'm dead set against it. So far, I'm hoping MacOS is at least starting to trend back. I still got a PowerBook G4 that works as well.