Changes which might not meet the quality bar to be a default user experience have a different calculus when they can be packaged up to be opt-in, and life-changing for certain people.

Global Accessibility Awareness Day also leads WWDC by a few weeks, so it serves as a taste of what Apple has been working on for the last year.

In terms of changes being led by a smaller feature, my favorite was night shift (which adjusted blue lighting at night) - which provided API months leading the first device with adaptive color adjustments from ambient light sensors.