This is the best summary I could come up with:
Google has been waging a "get the message" campaign against Apple for the past year or two, imploring the company to adopt RCS.
Last year, Apple CEO Tim Cook was asked on stage if the company would make messaging with Android better, and he responded, "I don’t hear our users asking that we put a lot of energy in on that at this point" and told the audience member to "just buy your mom an iPhone" if he wanted easier communication with his mother.
The Wall Street Journal ran an article last year subtitled "Teens Dread the Green Text Bubble," detailing the bullying that Android users were subject to due to SMS fallback dragging down the capabilities of iMessage group chats (87 percent of US teenagers have iPhones).
On the Android side of things, companies have been desperate to work better with iMessage, with Google hacking together an emoji response solution for Google Messages and Android manufacturer Nothing planning a wild "hack into iMessage" plan by running messages through Mac computers hosted in a data center.
Google has a few extensions on top of RCS that add important features like encryption, but that's not part of the GSMA standard.
Apple's statement carefully announcing support for "the standard as currently published by the GSM Association" seems specifically crafted to exclude all the fancy Google extras.
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