So it's basically a membrane keyboard with plungers hitting the "contacts" which are the keys on the touchscreen?

My first concern would be tolerances - enough press to feel good, but not so far that you have to press too hard. Too short and you might accidentally press keys you didn't mean to.

These are standard tactile dome switches. And the PCB pads are static.

I recall an old Ericsson phone that did this?

Do you have a model/name/link of this? I'm having a hard time following the description but it sounds like it sits on top of part of the screen? What happens when you fullscreen a video or otherwise don't have the keyboard up?

Seems to be this https://wonderfulengineering.com/samsung-unveils-a-qwerty-ke...

This, or similar. https://m.youtube.com/watch?v=mjwAqDS6EcU

It clips on the front of the phone. Has a small magnet that presumably triggers the onscreen keyboard to show and resize the visible screen.

My guess is 'you didn't look hard enough' or 'there were some but noone bothered to buy them and everyone stopped making them'. I was an avid sidekicker until the iphone2. I might have (lo 15 years ago) been interested in a slide out horizontal keyboard case for the iphone. Never happened, adapted and now with 'slide' keyboards never going back to chicklets.

I’ve seen various phone keyboards over the years but never recalled one that used capacitance.

I've certainly had little stick on joysticks (suction cup) that use capacitance, they sucked. I think I remember seeing similar keyboards at the time.

I'd be surprised if Sony Ericsson didn't have a patent on it, because that was exactly how the numeric keypad worked on their early smartphones. They were resistive displays though, so they just required something hard on the back to register a touch.