> Users who exceed 50 GB of data usage (across all their devices) will have their data speeds slowed to 256 kbps for the remainder of their billing cycle, unless they opt out and choose to be charged for additional full speed data usage at $10 per GB.
According to Google, it looks like that "unlimited" plan is 50 GB and then slowed to 256 kbps (basically useless). It isn't unlimited with lower priority after 50 GB. It's basically just limited to 50 GB given how useless 256 kbps is.
It's not common with mainstream US carriers on phone plans. It's common with MVNOs (virtual network operators).
On phone plans, carriers generally let a customer use unlimited low-priority data. If it's 3am and no one is using the network or you're in a location with plenty of capacity, your speeds are good. If there's a lot of people using the network, your speeds can vary since others get priority over you.
There's a big difference between "we prioritize other customers before you" and "you'll never exceed 256 kbps." The first kind still feels like unlimited for most people. It might mean getting 5 Mbps instead of 25 Mbps when there's a lot of network congestion, but most of the time you're just getting regular speeds and don't notice anything. I'd certainly notice 256 kbps.
MVNOs often throttle connections because they're usually paying for data on a per-GB basis from a network. Carriers can give their customers as much data as they want when the network isn't congested because it doesn't really cost them anything if you're using what would otherwise be unused bandwidth.
