Netflix recently noted that it has been throttling data on carriers like AT&T that have overage charges, but that was just a temporary measure. The company had planned to roll its mobile data saver feature out to everyone in May, and here we are. The feature should be live for everyone right now, no update needed.
The mobile data saver made a brief appearance for some people a few weeks back, but the version now officially deployed is much more robust than a simple toggle. In your app settings, you can either set data saver to automatic, or manually set the quality. Your options are off (no mobile data), low (4 hours per GB), medium (2 hours per GB), high (1 hour per GB), and unlimited. The unlimited setting is only recommended if you have an unlimited data plan or are particularly self-destructive.
Netflix says the default setting is around 600Kbps (3 hours per GB), which was the speed it throttled some carriers to previously. That should be enough for acceptable stream quality, assuming you're not too picky. The throttling on Netflix's end won't be necessary anymore now that this tool is in place. So, choose carefully.