Would also love to hear what you mean by “smart invisible regex,” sounds like AI slop to me.

I have launched apps focused on a new feature in the latest OS and regretted it. The # of people who have the latest OS is much smaller than the full install base for much longer than I thought. As a result, my marketing conversion was unnaturally low - people who liked the app idea but couldn't install because they had the wrong OS. This causes two problems: potential users I activated but couldn't convert and this signal gets internalized by the App Store, pushing down future impressions.

Now I always have a fallback implementation of the feature so I can target the prior OS. Both Mac and iOS.

Green, yellow, red or whatever hue is fine, as long as it's dark or light enough. Colorblind and non-colorblind people can see how dark or light a color is (luminance), but they might not agree on the hue. That's why WCAG contrast checks require luminance contrast and not hue contrast.

It's best to use a contrast checker because it's not always intuitive how dark or light a color is e.g. yellow and lime are almost as light as white.