I do not think it is Google's fault, at least not entirely. On stock OS', with the carriers here, almost everyone has RCS enabled and didn't even know it. When I was on stock Android 14, it only connected when I verified my number.
There seems to be something stopping the ACS URL from populating behind the scenes. This could be a block by the OS and/or the carrier failing to push the settings out when the device requests. The latter doesn't seem likely when there are so many individuals connected without challenge.
When I used the older version of Messages, I still had to use the Activity Launcher to manually enter the carrier ACS URL; it didn't populate on its own. Only then did the app ask me for my number, like Telegram or WhatsApp or Signal. The G- code was received, and it was automatically entered; however, it claimed there was an error and to try again. I ignored this message and immediately at the bottom, it said that RCS was registered to the carrier/sim card name.
Chats were switching to RCS messages, where applicable, some with a little lock and some without. When I connected to Mullvad VPN, the status changed to connecting. I looked at the status under debug mode, and the issue shown was due to a disconnection of the network; it did not reconnect until I disconnected the VPN. I can say that it is working since I added Messages to the split tunnelling list and connected.