I believe they still have the "Custom Plans" available, but they've made them harder to find.
I have a USMobile account with auto-pay disabled, and because auto-pay is disabled, each month, when I renew my plan, I have to select it all over again like I'm a new customer. I just renewed today, and it was a little harder to find -- I had to click on something like "low data user plans" before I could find the option to configure a voice-only account for another month. But I was able to get another month of my $8/mo voice-only service.
It's possible that I'm grandfathered in, but I suspect the renew interface that I have to use is the exact same interface as the configure-a-new-account interface, and I suspect that the custom accounts are still available to anyone.