I haven't seen this mentioned, and people are always asking about it, but my understanding of why Apple discontinued (PRODUCT)RED is that they were unhappy with the amount of money that was actually going to charitable causes. The number I heard floated was less than 10% was going to charity vs investors in 2022, the year Apple last released a (PRODUCT)RED iPhone. As I understand it, Apple now supports the charities directly instead.

Perhaps someday we'll get confirmation as to why it went away

@stroughtonsmith that sucks. And also because I loooooved that color.
@jlharter It was really striking, wasn’t it? I have a (RED) iPod Touch. It isn’t so useful these days, but I love how the anodized back looks on it.
@johnwells I went to an Apple Store recently wearing my (RED) Apple Watch. An employee stopped me and said, "Whoa, wait...where'd you get that color?" He had no idea they used to sell them. I've not updated since because I l loved the red so much. I now wear an analog watch as a result!