Yeahh, I 100% understand where you're coming from, a friend/user tried to add emoji support a few years ago unsuccessfully. It's a strange limitation I'd like to work around. I'll look into seeing if forbidding emoji explicitly is possible. Detecting emoji is a bit strange since a single emoji is actually composed of several unicode characters from what I remember from the last time I looked into this.
Just as some future notes for myself, I found some stuff here:
https://stackoverflow.com/questions/28023682/how-do-i-remove-emoji-characters-fr...
I made STM with dialogue text in mind, so there's not a lot in terms of features for text entry. It's something I've been wanting to flesh out for a while, but I figure Unity's built-in UI text is fine for text entry, since it won't need any special formatting and has everything configured already. I would like to get STM working in the same way, though... but I can't really make any time promises unfortunately, sorry.
So hmm... maybe that above code could be applied to text before it's sent to STM anyway...? I'll experiment with it soon!