Why don't dental chairs have a keyboard for easier communication?
In this way, you could more easily communicate with your dentist via the keyboard and a speech synthesizer.
Additionally, you could even have small talk throughout the procedure.
Ignoring that, IMO, very few people would use it, and that the dentist may have other things to attend to than the patient’s smalltalk, there’s the issue of having to clean it between patients. That would probably mean having a keyboard with a cover that one cannot easily blind-type on, further decreasing the number of people who would use it.
It would need to be able to handle saliva, blood and other fluids but maybe a low tech approach would be laminated cards. Yes, No, Not Sure, Stop, Suction Please, Am I supposed to be seeing colors?, Code Brown, (1-10 for pain scale), etc... Perhaps put raised dots or cut micro-notches on the cards so you can pick the right one without looking. Add a metal grommet in the corner and put them on a dog tag chain.
I don't ever plan to distract my dentist.
There is zero chance that I'd be able to effectively use a keyboard when the dentist is at work on my mouth.