If you don't receive iMessages it will automatically fall back to SMS (unless the sender specifically turned that feature off).
While neither mechanism is perfect, the RCS model treats lack of delivery to the handset as a potential problem, whereas iMessage ignores it. iMessage assumes your phone is just off, which for most people you are texting, is an unlikely scenario.
For users who actively turn off their device for certain activities, you have the opportunity to just wait, and when it arrives the error will clear, or send it as SMS and it will pick it up when the handset does turn on. But while the message is in flight for any length of time, the UI treats it as "something is wrong".
This means whenever you return to the conversation to check for a response or text more, you'll be reminded strongly that the message never arrived for them. And you always have the opportunity to resend on SMS during that time.
See https://discussions.apple.com/thread/8063349?sortBy=best
1. The sender cannot connect to the imessage server 2. The receiver does not connect to the imessage server, on any of the devices registered to receive at that particular address.
I know both methods happen since I didn't used to have data and I would receive SMSs when someone sent me an imessage and I was out and about.
It is self-seviced https://selfsolve.apple.com/deregister-imessage/
And what you link to is what I meant by
> assuming you forget to go through Apple's iMessage deregistration system.