RCS is an open standard. However, on Android you can only use it with Google chat. So android stops any other apps from using it. Nothing to stop you making your own phone from scratch and adopting it.
Samsung messages app also supports RCS, depending on your carrier, though? It's super fucking buggy and frequently switches back to sms so I still switched to Google messages, but it does technically have it.
Nothing to stop you making your own phone from scratch and adopting it
Unless we count current market state when it's not possible to make slightly different phone, as even camera or modem manufactures do not publish documentation for those components and just give binary blobs for Android.
And forking Android adding deep integrated features without Google will be a pain.