You can use any type of proxy over 5G, it's still just internet after all. The difference between 5G and 4G is that 5G can have much higher theoretical speeds at significantly lower latency than 4G, the performance can be similar to presently available fiber optics. 5G proxies won't be necessary for a great while, if you want mobile proxies, 4G should suffice, as it was mentioned above, those are still quite expensive.