SMS is not an instant messaging protocol either.
Yes, it is. It was always an instant messaging protocol. It may not have been called instant messaging, but that was its intended purpose - initially for carriers to send alerts or other notifications to their customers. Instantly.
The very first SMS was quite literally an instant message from one person to another, saying "Merry Christmas". In '92. Every Nokia device supported SMS by '93, and many more manufacturers were not far behind.
Sometimes they arrive quickly, sometimes they don’t arrive at all. :/
Occasional technical hiccups (e.g. not received) do not change the originally-intended function. I've had more issues sending/receiving RCS messages than SMS/MMS messages.