It’s not a weird caste system. It’s just that people have always primarily just used SMS in the US, and if the people texting all happen to have iPhones, then there are some extra features tacked on (from the perception of the end user). Having been in many many large group chats for various activities and events, where it’s never 100% apple and just SMS, absolutely nobody cares at all. It’s just that maybe some teens and tweens use the colors to judge and exclude, which they famously find justifications for doing in every generation, and probably even that is overblown by the media.
There simply was never an incentive to kind of force everybody to move over to e.g. WhatsApp, and people don’t bother to do something like that en masse without a need to.
“I don’t really like X, but as long as they don’t then do Y I guess it’s okay”
They always do Y