There's nothing stopping folks from typing a message an LLM wrote one at a time, but the idea of increasing the human cost of sending messages is an interesting one, or at least I thought :-(
For instance, the employee at Apple that decided to pull ICE Block from the store could decide that the "admissible in court" bit should be false if it looks like a police officer is in frame.
Similarly, the keyboard could decide your social credit score is too low, and just stop attesting. A court could order this behavior.
Or, you could fail mandatory age / id verification because your credit card expired, and then all the above + more could happen! Good luck getting through to credit card tech support at that point...