So it's not required that it's CO2 or R290, but rather than you can use any refrigerant that has a GWP of 1 or less.

R290 seems an ideal refrigerant, especially since the technology to use it as a refrigerant is so old/mature. The challenge is that it's /highly/ flammable. This is true of most refrigerants with a GWP less than 1. R1234yf partly exists because of a desire to reduce flammability.