Yes, The phones are tested @ 2TB
Yes, prototypes of the cards do exists.
We dont see these cards in the market for a number of reasons.
A phone company cannot simply claim support of certain memory without ever testing them. It might not be with an actual size microSD card but you can connect the card interface to another memory device of that capacity to test it. Its like taking a AA battery, hooking it up with wires, and connecting it to a device with AAA battery slot. It will work.
A 2TB SD card prototype does exist, but it costs too much to manufacture. Not many people would spend near about $1000 for one, so the company would not start manufacturing them till they have enough market share, their materials become cheap or the technology is ready. As we move ahead in time, 8K and so on formats will start becoming the norms, technology will evolve which would enable us to read and write to a device at a much faster speed. Then and only then it becomes sense to have such high memory cards. So companies need to wait before they invest into them. Also, the card needs to be tested extensively before they are released. There will always be x% of cards which will fail, does the cost of replacing them justify the sales.
There are many such reasons.