For many years now KDE has focused on polish, bug fixing and "nice-to-have" improvements rather than major redesigns, and it paid off.
I often wondered why desktop UIs became so terrible somewhere in the 2010s and I don't want to attribute it to laziness, greed, etc... People have been lazy and greedy since people existed, there must have been something else. And I think that mobile is the answer.
UI designers are facing a really hard problem, if not impossible. Most apps nowadays have desktop and mobile variants, and you want some consistency, as you don't want users to relearn everything when switching variants. But mobile platforms, with their small touchscreens are completely different from desktop platforms with their large screens, keyboards and mice. So what do you do?
In addition to mobile, you often need to target the browser too, so: native desktop, native mobile, browser desktop, browser mobile. And then you add commercial consideration like cost, brand identity, and the idea that if you didn't change the UI, you didn't change anything. Commercial considerations have always been a thing, but the multiplication of platforms made it worse, prompting for the idea of running everything in a browser, and having the desktop inferface just being the mobile interface with extra stuff.
Who do you think has been "infected" by the "mobile" virus? KDE's only real competitor is way more keyboard focused than KDE...
[1] Hamburger menus are designed to make efficient use of a small vertical display where horizontal screen space is a limited commodity, which just is not the case at all for a large horizontal computer monitor. On a large horizontal display, they're a straight downgrade since you need to click the menu to see what's inside it, which makes action discovery harder. This click is also added to a lot of actions so they add more friction to almost all interactions.
Supposing I did, the only hamburger menus I can think of contain lesser-important functions of each app, like seeing the version/build number, or certain settings. I'm not sure I want something like a "See hidden files" ticker occupying screen real estate forever when I could just set it once in an accessory menu.
I question whether these critiques would evaporate if, instead of the three horizontal bars, Gnome instead used a gear icon or something, and turned their contents into a pop-up window rather than a popover element.
gEdit places almost everything in the hamburger menu; opening and saving files have dedicated buttons but for example find/replace is behind the burger, as is "save as". It may not matter much if you use keyboard shortcuts (ctrl+f is pretty common for find and I never try to look for it in the menu) but one might still expect a GUI to allow its features to be easily accessed without the use of a keyboard. I don't think the mix of a few dedicated buttons and a single hamburger menu is necessarily good for discoverability either.
The Image Viewer puts file management and image rotating in the hamburger menu. Oddly enough, other image editing options are available in a separate editing mode that's accessed via its own dedicated button. Also, although file management features are behind the hamburger menu, for some reason image properties are behind a dedicated button.
In both cases the only reason the hamburger menus aren't more populated is because there just isn't that much functionality in either app to begin with.
Evince (the document viewer) also puts almost everything in the hamburger menu -- although in that case, if a traditional menu bar were used instead of the hamburger, most of its functions would probably only be split between "file" and "view" menus or something along those lines.
I'm not sure if those apps are still Gnome defaults but they're some of the examples of what I'd consider somewhat poorly considered use of the hamburger menu.
Outside of Gnome, the new UI in JetBrains IDEs has switched to hiding typical menu bar menus behind a hamburger menu button. I honestly don't understand that decision at all: the menus are still the same, they just require an additional click to access, and since the selection of available menus is only revealed after clicking the button, you can only start scanning for the menu you're looking for after the reveal. While separate from free software desktop design, the new UI in those IDEs is another example of what I would also consider mobile-influenced degradation of desktop UIs -- and a particularly weird one at that.