Pix Wireless is sending out emails telling subscribers of their Verizon/Silver plans that the plans will be cancelled on 9/20, and that they need to port out or make other arrangements.
Pix's website also indicates Sprint (Yellow) and Indigo (T-Mobile) plans are not currently available.
This leaves AT&T (Emerald) as the only current offer of the big 4.
I do see that Pix is now offering a World SIM, and a VoIP calling plan/app. I don't really know if they're new. (The VoIP plan is not free.)
Perhaps the Sprint & T-Mobile plans are just up for contract renewal, and might return.