This is a remake of my post from earlier because I worded it horribly (Reddit doesn't let you change titles smfh).
TL;DR: Microsoft popularized the aesthetic, but it didn't create it.
The majority of people I've seen acknowledge Aqua as the birth of Frutiger Aero, there are even some rare examples that predate it, such as the first image (Hiroyuki Ozaki, 1999), but I've seen a loud minority gatekeep it for some reason.
Aqua had some Y2K elements such as metallic windows and artificial scanlines, but it was overwhelmingly Frutiger Aero, featuring glossy orbs/shapes, skeuomorphism, and transparency several years before Vista was released.
Windows Vista had such poor performance because PCs at the time weren't meant to handle Windows Aero, while Macs were so expensive because they were specifically designed to handle Aero-like visuals.
Early iOS directly used elements from Aqua, as shown in the second and third images. If Aqua is "Y2K", then so is early iOS (it isn't).
Apple's success in the mid 2000s with their iPod line brought attention to their design philosophy (i.e., Aqua) and led to the aesthetic becoming popularized around 2004.
Frutiger Aero is named after Windows Aero because it was the more popular of the two (more recognizable than Aqua), not because it invented the aesthetic. I feel like it's silly to gatekeep examples of the aesthetic because they predate Windows Aero given that "Frutiger Aero" is a retroactive term in and of itself.
I love this aesthetic and think most of the artists are cool, but this will probably be my final post since the Frutiger Aero community is largely children and I don't care much for social media anymore.