SMS has supported Unicode since the very early 2000s. I remember getting SMS messages in Thai on my Nokia 7110 GSM when I was in Thailand in 2003. So there's no technical reason why iMessage fallback to SMS doesn't support emoji.
I'm not talking about the ability to put emojis in text messages. I'm specifically talking about the way virtually all messaging apps these days (iMessage, Android RCS messaging, Slack, etc.) have the ability to do emoji reactions to previous messages, e.g. long press and add a thumbs up, heart, barfing, whatever.
