I am guessing you are referring to Dropping iOS 15 support in May 2025, as @merefield said, the new minimum supported version will be iOS 16.
As to the why: it is very time-consuming to support old operating systems. It is costly for us, every time we want to use new browser APIs in CSS or Javascript, the old OSs block us from doing so. For example, the announcement mentions a CSS feature, subgrid, which we’d love to use because it can simplify (and improve) layouts in Discourse core and themes.