The iTunes icon in 10.7 kicked it off. If felt lazy and out of place, even at the time I hated it. Then other new apps followed in the same style... App Store, Books, TV, News... By 10.15 most of the life had been drained from the icons. Then in 10.16 they went fully corporate and boring, shoving them all into their little squircle jails where they've remained for 6 years and counting.

It provides no info about what windows are open, it does random things when you click on an icon, it has an indicator that tells you that an app is in memory but may or may not have a window. And all these become even worse once you have multiple monitors / spaces.

Just copy whatever gnome/kde/windows are doing. All of these have figured it out. Only macOS needs an investment of like $50 per year in helper apps to deal with the poor app and window management.

What I do know is that in the uniform-looking icons we have in the current dock, I seem to find myself hunting for the right icon to click a lot more than I have in the past.

This icon goes back to the Mac Clone era when they wanted separate branding for System 7.5 etc. Kinda funny it survived Jobs and is still around.

Also, IMO there's a lot of undue nostalgia for Snow Leopard. But that 3D Dock was cool. (It didn't do that when you put your dock where it belongs, on the right side.)

I’ve come to thoroughly dislike the grind getting rid of them all with the dog slow animation each new Mac I set up.

10.10 onwards is pretty forgettable.

During the 10.5-10.7 era there were a number of third-party dock themes people made that worked by swapping a few image resources, which in addition to visuals could adjust reflectiveness. I had mine set to a nice studio-lit dark gray matte shelf with white 3D indicator arrows.

defaults write com.apple.dock autohide-time-modifier -int 20

so that I never have to see it.