The new Gimp icons are fine for experienced users (like myself) who only need a faint pointer to the function if the tool. I use shortcuts for most of them anyway.
But users who are only starting to learn the ropes the classic big and colorful icons are a serious help. They are still available, just not made the default.
I use GIMP enough that I would call myself experienced, but I don't generally identify icons by shapes, I'm used to identifying almost all icons by color.
The half-red half-blue thing was the eraser. The yellow thing with a black dot was the clone stamp. The yellow stick was a pencil, the brown stick was a paintbrush, the red stick was an airbrush, the dark grey stick was the magic wand selector, the black bomb was the burn tool, and the white 45-degree square was the fill tool. The black A was the text tool, the blue A was the measure tool.
I'm reasonably experienced with GIMP and I find them useless. It's seriously difficult to quickly pick out the icon I'm looking for even if I know what it looks like. Even months (years?) after they changed the icons I was still finding myself spending seconds or literally tens of seconds sometimes trying to find the right icon.
Anyway I realised you can change them back and now finding things is almost instant again. Eraser? -> Look for pink -> done.
But users who are only starting to learn the ropes the classic big and colorful icons are a serious help. They are still available, just not made the default.