Earlier quoted context omitted.
SMS messages have always been green on iOS. Since before iMessage existed. I don't recall ever having trouble reading them.
They deliberately reduced the contrast. Compare iOS 5 when iMessage came out: https://cdn.arstechnica.net/wp-content/uploads/archive/09-27... and now: https://images.macworld.co.uk/cmsdata/features/3468389/how_t... This underscores GP's point: Apple is expert at making anti-consumer decisions that fall just inside the Overton window.
In general, Apple has lowered contrast throughout the UI over the years. There's an accessibility setting for high contrast if you need it.