Apple is the last major RCS holdout, as U.S. carriers that include Verizon, AT&T, and T-Mobile have already started adopting RCS support for Android devices.
My understanding was that Verizon was targeting a 2022 roll-out. Most of the support has been via Google hosting the RCS server. I suppose, quite strangely, Apple hasn't wanted the Android team to run their chat infrastructure.
Apple has not commented either way on whether it plans to add RCS support in the future, so the company's position is still unknown.
Fill in the blank with RCS. Apple Corporate policy is not to talk about the features of products which are not released.
That said, any competitive advantage that Apple has in chat is mostly Google's making. I have literally lost count of the times Google has screwed over their base by rolling out and deprecating various chat offerings.
After building and burning a dozen bridges with users, Google's one chance of people giving a crap about Google's chat offerings at this point is by piggy-backing on RCS adoption. And the once chance at this point of RCS getting adopted is if it works between everyone and with zero setup.
The US carriers dropped their plan of shipping their own RCS app on Android likely due to the mutual desperation.
That said, Google already has extensions on RCS (such as the end-to-end encryption support, which only works with Android Messenger). Their encryption is inferior in many ways to the other encrypted chat apps such as Signal and iMessage - it only supports 1-to-1 chats, and my understanding is that users must manually verify that they have the same "verification code" out of band to know that the chat is actually encrypted.
It also was first published in June 2021 - meaning even if Apple wanted to support RCS in an iOS release and this e2e support pushed them toward adopting it, that it came way too late for iOS 15.