I don't expect all, or even any, of these changes to be approved but they might spark some discussion.
The round circles behind some icons are made a bit larger so they are much less likely to overflow at the edges, as noticed with Collection, Mesh, Mesh, etc.
The open/close triangles are drawn with the crisp vector code we use elsewhere, like in Properties, rather than the fuzzier icons.
When multiple icons are shown on a collapsed row there is a little vertical separator. That was always drawn darker on all backgrounds. With this patch it uses the text color (at lower opacity), which matches better as the theme changes. And is narrower so it is a little less striking.
The hierarchy lines are removed. They never did much good. And the lines were visible *through* the icons, which looked terrible. In the capture below you can see the line inside the Collection icon. I'm sure some people loved them though.
Selected rows did not match up quite to their target rows, leaving black lines around it. This makes them fit better.
Subtle changes, except for the loss of the lines, but you can see here before (left) and after (right):