It's more than a proof of concept; there are lots of examples, unit tests, and the linked documentation.

Pico Keys (https://www.tindie.com/products/fabien-c/pico-keys/) is a good example of a complex project built with rp2040_hal.

Note that this library is specific to the RP2040, not the larger Raspberry Pi boards. Some Ada drivers for those exist, but are not as well documented.

I also gave a short talk about some of the challenges I encountered earlier this year: https://youtu.be/Pwd9Q5ELAGM