Craig Grannell:

The standard macOS interface has quite a few semi-transparent elements, which like frosted glass provide a glimpse of what’s beneath them. At Apple events, execs go giddy about how pretty this is. In use, these elements vary from being distracting to outright dangerous. For example, if you have a motion-sickness issue and an animating web page is sitting behind a semi-transparent element, it can take a while before you realise it’s affecting you, by which time it’s too late and you’re already dizzy.

And:

“Fine”, says Apple, grumpily, “so just turn on Reduce transparency”. Only it’s not that simple. Because when you do, Apple designers get in a strop and hurl logic out of the window. What you’d expect to happen is for macOS to remove the semi-transparent bits. So instead of Finder sidebars or the macOS app switcher showing what’s beneath them, they’d just have a neutral solid background. Nope. Instead, in its infinite wisdom, Apple’s decided those components should instead be coloured by your Desktop background.

Stephen Hackett put together a few screenshots to show off this effect.

To me, this sort of thing happens due to lack of a specific branch of testing. Seems to me, someone at Apple should reach out to Craig Grannell and ask him (and other leaders in the accessibility community) to beta test new software/hardware early in the cycle, so they have time to address these sorts of issues. I believe accessibility testing would be greatly enhanced by voices, hands, eyes of real world experience.