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